Scenario #1: You are a rogue who has ended up in the midst of combat with multiple foes. Before your turn comes, there will be 2 people attacking you. Let's say in this case, the first one is a Wizard that uses Fire Bolt cantrip on you and hits you, but rolls a 1 on damage. Next, a Path of the Berserker subclassed Half-Orc Barbarian runs up to you and crits you with his first attack with his Greataxe, getting Savage Attacks racial which increases his crit damage by another die roll, and potentially getting Brutal Critical class feature (if the enemy is 9th level or higher) for at least one more die roll of damage.
As a rogue, you have a class feature called Uncanny Dodge. This means you can (read: not must) use your reaction when an attacker you can see hits you with an attack to reduce that attack's damage against you by half. With a preemptive "set it and forget it" system where you use up your reaction the first time it is available to be used each round of combat, in this scenario you would use Uncanny Dodge on the 1 damage Fire Bolt cantrip, effectively wasting the ability. You could have used it to reduce the damage of the massive Barbarian Crit that is probably going to one-shot you to 0 HP.
No.
You don't know how much damage you take when you use uncanny dodge.
Uncanny Dodge p96 Player's Handbook
Starting at 5th level, when an attacker that you can see hits you with an attack, you can use your reaction to halve the attack's damage against you.
Rules as Written: Rogue chooses to use Uncanny Dodge when they get hit, but before the damage is rolled.
Scenario #2: You are a Cleric that went down the War domain path so that you could wear heavy armor and use martial weapons and get into the thick of the fight. Because you plan on using either a 1h and shield or using a 2h weapon, you decide to take the War Caster feat. The War Caster feat allows you to do better at concentration checks to maintain spells, allows you to cast spells that require somatic components (a lot of spells do) while having both hands full with said shield or 2h weapon, and additionally allows you to use your reaction to replace an Attack of Opportunity with a spell that has a casting time of 1 action that targets only that creature. This is a very common feat to get on "battle caster" type builds.
We have no idea if this feat is even implemented in Baldur's Gate 3.
As such using it as an example of how it makes a completely different implementation of reactions necessary is silly.
Early access is 35 days away.
Speculating on imagined edge case scenarios from a build posted 40+ days ago does not seem to be very valuable feedback.
Those of us who are playing Early Access will provide plenty of feed back on the build that we get to play.
If you choose not to participate in Early Access I'm sure lots of videos will be posted that will show you how the feature plays out and you could provide more meaningful feedback then.
