While writing the BG3 fan fic, I took all the characters to level 6. So, let's just take 4 of them. Astarion, Wyll, Gale and Karlach (I made her a paladin). They're fighting against a Drow Wizard, a goblin captain and 3 other goblins.
Popup Window Version
Round 1
Astarion's turn. He takes his turn by running up and attacking the Goblin Captain. Goblin 1 charges at him, attacks him and hits dealing 8 damage. Popup. "You've taken 8 damage from Goblin 1. Use Uncanny Dodge Yes/No?" I choose No. Goblin finishes his attack, uses a Bonus to Disengage and rushes up to Gale. No 2nd popup because he used Disengage.
Drow Wizard's turn. He casts Fireball. Popup. "Gale/Wyll. Drow Wizard is casting Fireball. Use Reaction to cast Counterspell Yes/No?" I pick Yes for Wyll. Window closes. Wyll is seen casting his spell which undoes the wizard's spell. Wyll has used his reaction.
Wyll's turn. He attacks Goblin Captain with Eldritch Blast and has Repelling Blast, sending him away from Astarion. He's done.
Gale's turn. He casts Fireball. Drow Wizard uses Counterspell. Gale is done.
Goblin Captain gets up, runs up to Astarion and attacks him. He hits. Popup. "You've taken 15 damage from Goblin Captain. Use Uncanny Dodge Yes/No?" I pick Yes. Astarion has used his Reaction. Goblin Captain doesn't need to use his Bonus, now, to Disengage. So, he runs up to Karlach and uses his Bonus to make an attack with his off hand (he uses 2 weapons). He hits.
Karlach's turn. She attacks and hits Goblin Captain, using Divine Smite Level 2. She misses and gets Divine Smite 2 refunded as if she never used it because she missed.
Goblin 3 attacks Gale and shoves him off a cliff. Popup. "Goblin 3 has shoved you off a cliff. Fall is 60 feet. Use Reaction to cast Featherfall? Yes/No?" I pick Yes and Gale takes no damage. He lands safely at the bottom. Goblin 3 runs past Karlach to get behind Astarion. Karlach gets Attack of Opportunity. Popup. "Goblin 3 triggered an Attack of Opportunity. Use Reaction to Attack Yes/No?" Options appear below the question. Attack without Smite Yes/No? Attack with Smite Level 1 Yes? Attack with Smite Level 2 Yes? I pick Yes for Attack with Smite Level 1. Window closes, and the goblin is hit by Smite 1. Goblin dies.
Scripted/Preset Reactions Version
Prior to battle, I - the player - pull up my Reactions menu to set all my defaults for my characters. Astarion has Uncanny Dodge, and it allows me to set the trigger in increments of 5. I choose 10. Anything from 0-9 damage done to Astarion won't trigger the reaction. Any damage of 10 or higher will. I then set his Attack of Opportunity to default to "Trigger On Any Enemy". He's done.
Wyll. He has AOO, Counterspell and Hellish Rebuke. I set his AOO to the same as Astarion's by default. Then I set Counterspell to only trigger on level 3 or above spells. Hellish Rebuke I set to "Trigger On Enemy Leader(s) Only." That way he doesn't waste it on just any grunts. (The game would have to tag who is the leader of every enemy group for this to work with perhaps multiple leaders being tagged. In the scenario below, the Drow Wizard and Goblin Captain would be tagged as leaders.) He's done.
Gale. He has AOO, Absorb Elements, Feather Fall, Shield, Silvery Barbs and Counterspell. He disables AOO, because he sucks at melee anyway. He disables Absorb Elements because he doesn't want to trigger that unless he knows there's a particular element he might be up against. He sets Feather Fall to 30+ feet of falling distance, so that only if he falls 30+ feet will it trigger. He disables Shield because he doesn't want it being cast if some random, baby enemy attacks him. He also disables Silvery Barbs because he needs to know his enemies for that one. Finally, he sets Counterspell to only trigger on level 3 or above spells. Popup appears. Wyll also knows Counterspell. Set priority. I set Wyll as priority so that Gale doesn't waste his reaction if Wyll is going to cast the spell instead. Let the Warlock who doesn't have Fireball cast it first. He's done.
Karlach. She has AOO and Divine Smite Reaction to set. She sets AOO as the others do, and she sets Divine Smite based on increments of 10 HP. Any enemies who have 1-9 HP she won't cast Divine Smite. 10-19, she uses Smite Level 1. 20-29, she uses Smite 2. She's done.
Round 1. Astarion's turn. He takes his turn by running up and attacking the Goblin Captain. He goes to his Reaction section on the UI and disables his AOO. He doesn't want it to trigger because he wants to make sure his Uncanny Dodge will if he gets hit hard enough. Goblin 1 charges at him, attacks him and hits dealing 8 damage. Goblin finishes his attack, uses a Bonus to Disengage and rushes up to Gale.
Drow Wizard's turn. He casts Fireball. Wyll casts Counterspell.
Wy'lls turn. He attacks Goblin Captain with Eldritch Blast and has Repelling Blast, sending him away from Astarion. He goes into the Reaction section to turn off AOO and Hellish Rebuke. He wants to make sure that he reserves his last spell slot for another Counterspell in case the wizard casts another high powered spell. He's done.
Gale's turn. He casts Fireball. Drow Wizard uses Counterspell. Gale goes to his Reactions and turns on Absorb Elements, setting it to Fire as the trigger. Oh... well... maybe lightning too. This guy could cast Lightning Bolt. He also turns on Shield and sets it to Magic Missile only just in cast the wizard tries to do Magic Missiles before Gale's next turn. Gale is done.
Goblin Captain gets up, runs up to Astarion and attacks him. He hits. Astarion does Uncanny Dodge animation. Damage should have been 15, but it is only 7. Goblin Captain doesn't need to use his Bonus, now, to Disengage. So, he runs up to Karlach and uses his Bonus to make an attack with his off hand (he uses 2 weapons). He hits.
Karlach's turn. She attacks and hits Goblin Captain, using Divine Smite Level 2. She misses and gets Divine Smite 2 refunded as if she never used it because she missed.
Goblin 3 attacks Gale and shoves him off a cliff. He casts Feather Fall and takes no damage. He lands safely at the bottom. Goblin 3 runs past Karlach to get behind Astarion. Karlach uses Smite Level 1 because the goblin has 11 HP. Goblin dies.
As you can see, both can work. The only question is, which method do you prefer? Do you like setting up the Reactions beforehand, or do you want popups?
Keep in mind that this scenario worked out very well for the Scripted/Preset Reactions Version. It actually flowed very smoothly and worked well. Not all scenarios would work out so well, but this proves that it could. (I did actually play this out via Tabletop, so it isn't just made up in my head. I legit wanted to see how both would play out.)
The primary issue I have with this scenario is:
If the scenario played out where the Goblin Captain went first, he might attack Wyll and trigger Hellish Rebuke, thus robbing Wyll of his Reaction. This would change the entire battle. Gale would then have to use his Reaction to do Counterspell which would mean he wouldn't have Feather Fall to use. With the Popup Version, the player could still say, "No. I don't want Wyll to use Hellish Rebuke because the Drow Wizard hasn't gone yet. I'd prefer to save it just in case." Notice how with the Scripted Reactions version, the player has no ability to do that. Because he set the Hellish Rebuke default to Leaders Only, if the Goblin Captain, who has the leader tag, goes first, Wyll will automatically use Hellish Rebuke on him. The player would have no way to reset his/her Reaction defaults because Wyll hasn't taken his turn yet.
This could, however, be fixed by creating a Pre-Combat Phase. As soon as Combat is initiated, the game allows the player to tweak his/her reactions based on the combatants the player is facing. If they created this phase, the player could then set Wyll's Hellish Rebuke and AOO to off. This would dramatically help to ensure that the presets go more smoothly in the first round. After that, the Reaction Presets would need to be adjusted each and every time the player takes a turn for each character, as described in the scenario.
Another issue I foresee with the Preset Version is that it still requires the player to remember Reactions. This would, however, come with time. Players would get used to the idea after awhile that they need to tweak Reactions before ending a turn.
Another issue is that an enemy might just barely meet a Preset parameter and trigger a reaction, even though you might not necessarily want such a thing to happen. In other words, I set my Hellish Rebuke to Leaders Only, and I think nothing of it. The Goblin Captain triggers it, but I didn't realize he had a leader tag. Dang! Now Wyll has no ability to Counterspell the Drow Wizard. If I had a popup, I'd be sure to not Hellish Rebuke the captain, and I wouldn't have to research my enemies to ensure I was setting a preset up right.
Finally, one of the biggest issues with the Preset Version... What if I set my Reaction to AOO off (or Wyll's Hellish Rebuke to off, or whatever), but the last enemy in combat WOULD trigger AOO? No one else wound up triggering any of my Preset Reactions that round. Well. Too bad. I lose my Reaction. I could have AOO'd if I'd gotten a popup that asked me, but because I set AOO to off, and it's all based on Presets, I missed an opportunity to kill an enemy. Situations like this would be the biggest downside of the Scripted/Preset Version. You might miss out on even using Reactions or you might still trigger some Reactions that you didn't want triggered.
I must say, the Scripted/Preset Version did flow so much better. The Popup Version did have a LOT of stopping points. Even if they are quick, it really does make combat a jerking, choppy mess. Every few seconds, another popup occurs, making it a lot less fluid. Meanwhile, the Scripted/Preset Version did pretty much the same thing without any pauses or breaks in combat.
The real trick with the Scripted/Preset Version is that you really have to get used to paying attention to Reactions and managing them beforehand. You need to make sure that you're not forgetting them and that you're really thinking through them in advance. It's kind of like RtwP Scripts for your companions. You have to go through and take the time to really make sure they are doing what you want them to do. And, if they give you a Pre-Combat Phase to adjust all Reactions before the first round of combat, that would be REALLY helpful.
The true advantage of popups is that you can think Reactions through as combat is happening, so you, the player, are literally reacting to something happening. It may be choppy combat, but it's full control for the player, and it's more 1st-Time Player User Friendly. 1st-Time Players who don't know a thing about Reactions and such, can get a popup to help them think through it and walk through it. There's nothing to really help the 1st-Time Player figure out how to best set up their Presets for Reactions in the other version.
I don't know. I go back and forth, honestly. I think Ragnarok is right, when all's said and done. Let the player have a "Reaction Presets On" and "Ask" options, with the game set to Ask as default. That way, if the player is new to the game, the default setting is "Ask" which is the Popup Version. For more experienced users, the "Reaction Presets On" is the Scripted/Presets Version which allows the player to preset all their reactions and creates the Pre-Combat Phase so the player can preset their Reactions at the start of combat before the first character takes a turn.