Uncanny dodge is clear, but only if you understand the style writing practices of 5e, which many (perfectly understandably) don't pay attention to until it matters. In this case, the wording on the ability is specific - it's when you are hit. That's a different trigger timing to "When you (or a target) is attacked" and it's different again to "When you Take Damage".
Three examples:
Protection Fighting Style's trigger is when a valid target "is attacked"; you must decide when the attack is declared; this means that you decide before the attack is rolled, and thus before you know whether it would be a hit or not.
Uncanny Dodge is triggered when you "are hit"; you decide when an attack hits you; this means that you do not have to use it before the attack is rolled, and so you cannot waste it on an attack that would miss anyway, but you must decide when the hit is confirmed, before the damage is rolled.
Misty Escape (Warlock fey pact feature) is triggered when you "take damage"; you decide to use it once you take damage - after the attack is declared, after it is rolled and the hit confirmed, and after the damage is rolled.
This is not a 'Dm discretion' thing, it's a RAW thing; as always individual Dms can rule otherwise - but by base rules, it's distinct and clear in its timing, and it's before the damage roll that the decision must be made. If the game tells you the damage first, then it is giving you a very prominent concession that makes the ability more powerful than it is meant to be - it removes a lot of the decision cost of whether to use it or not - however, I'm happy to concede that this may not be a bad thing for a video game setting/translation.
I understand what you're saying, but I will counter with two points:
1. Other reactions specifically say "before dice are rolled" or "before results are revealed". This reaction does not. Thus, if one wants to get REALLY technical, they can argue that the wording does not say "before damage dice are rolled". It simply says "if a rogue is hit by any attack". Although I agree with you that the meaning of what is written is PROBABLY that it should be before damage is rolled, there are A LOT of posts online with people arguing whether it should be before or after damage dice are rolled especially since many players and DMs roll damage dice WITH attack rolls. Also, plenty of examples exist on various posts of people saying that they usually save Uncanny Dodge for when an enemy Crits them or does a high powered attack. So, I would say that since there is so much room for debate on this topic, it is not "clear" that Uncanny Dodge should be decided immediately after being hit but before damage is rolled.
2. Even IF we had a WotC guru pop on and say, "Nope. Niara's totally right. 100%. In Tabletop, Uncanny Dodge is supposed to be decided right after being hit but before damage is rolled," what I'm suggesting is that in order to make presets work well to minimize or even negate the need for a prompt for Uncanny Dodge every time a rogue is hit by an enemy, Larian COULD have a preset option based on Damage Done to the Rogue. It wouldn't be a HUGE change in the overall mechanics to make it so that players aren't interrupted every time a Rogue is hit with a prompt/popup. ALSO, it works to the benefit of the player, not the enemy NPCs. Let's face it, you know the AI is going to work however the AI is going to work with their rogues, probably using Uncanny Dodge whenever damage YOU deal is high, so why not make it so that players get the same benefit - and even IF the AI is playing where it decides before damage is determined, somewhere in their algorithms, would it be so terrible if the player is given a slight advantage of allowing them to only trigger Uncanny Dodge if damage is greater than a certain value? It's really not a game-breaking mechanic that I'm suggesting, and it could seriously cut down on how often prompts/popups occur.
And, in terms of Uncanny Dodge, I use this example because in Solasta it is one of the prompts/popups I receive more frequently with my rogue character after he gets hit. I'm also thinking about battles like the goblin camp that has 30 enemies swarming your 4 characters. Yes. Absolutely. Your rogue is going to be a frequent target out of 4 PCs. 25% probable chance that the rogue will be targetted, and out of 30, that's almost 8 enemies in a single round. If my rogue is close to a bugbear or an ogre, I'm probably going to say No to most of those prompts until the ogre or bugbear comes and hits my rogue. GRANTED, it's ONLY going to trigger a prompt if the rogue is hit, so it's not like there will be 8 prompts, but my point is that IF I can set a preset that says, "Only if the damage is more than 10 will Uncanny Dodge trigger" then I may not even get a single prompt that entire round. Even if Goblins 2, 4, and 10 are the only ones to hit, if they only do 1-9 damage each, I'm good. I didn't waste my reaction on Uncanny Dodge to reduce damage in half for something so petty. But if Goblin 10 hit and criticalled me for a total of 18 damage, yeah... I might want to half that to 9. With the preset, that would be like 1 prompt. "You just got hit for 18 damage by Goblin 10. Uncanny Dodge?" Yes please. I got hit 3 times, but only the last was worth my time. Absolute Prompt system? 3 prompts that round. Preset using damage? 1 prompt that round. 1/3 of the total prompts used simply by a slightly more intuitive preset system that I, the player, can set before combat or even during my turn if I want to.
