Rant spoiler-ed below, because its mostly just complaining, but it feels like a lot of games lately have added 'gamified xp' to their systems, and I don't understand the appeal at all. The most positive any player I've talked to has felt about the systems are 'meh, I could take it or leave it', with most slightly disliking them.
So, mostly, I'm looking for people that like these kinds of mechanics (I have to assume they're out there if so many games feel like they're adding them), and what you is it you like about them.
But I feel like Milestone XP just makes the most sense for any traditional coop party RPG.
rant
I like trying out a bunch of different systems, and there's a lot to like, especially with a lot of the smaller games out there. I do like there 'narrative' approach ttrpgs feel like they've been taking recently, but between Chronicles of Darkness, ICON, Forged in the Dark, Apocalypse world, etc., and all there spinoff systems all having XP be earned for specific actions in game, its just a pain in the ass, that takes me out of the action.
I mostly GM, but I strongly prefer milestone XP. For some of the above games, its easy enough to gut their bespoke XP systems out of it, and just have players advance over time, but in several of them, it fucks with the overall balance, since several actions are 'bad', but made worthwhile because they earn XP.
But I don't like the feeling of interrupting game to award point, and adjudicate character advancement. And I hate systems that have players advance unevenly. There's always going to be a certain degree on uneven-ness in player attention, of rules mastery, and of spotlighting. As GM, its important to manage those so that everyone gets a chance to shine. But it just feels like increasing that workload for the GM, to need to additionally pay attention to the XP, and try to drag or XP share the players that don't find those systems engaging to not mechanically fall behind.
And as a player, I tend to have higher system mastery and attention than the others I play with, and tbh it feels bad to end up with more XP because of that. But also, it feels bad to knowingly pass up free XP by purposefully not engaging with those systems.
Really, only Call of Cthulhu and Paranoia feel like their gamified XP fits, and that's largely because the games are supposed to feel hopeless and unfair, and in the case of Paranoia, gamified and playing favorites.
I play a lot of OSR, and even in those games I prefer milestone. It just feels so much better - as a GM, I know exactly when the party will level up and I can prep encounters accordingly, as a player getting a level up after finishing a storyline or something feels really nice (even better than splitting treasure IMO). That said I don't mind if player advancement is a bit uneven, especially if that unevenness is one of the ways classes are balanced against each other, so in my games I usually give out XP in lumps of a thousand or so every time the party finishes doing something.
Even in the computer world, games like BG3 would benefit from a milestone system. I remember getting to the end of Act 1 still at level 3 and getting trounced the moment I tried to move on, necessitating me going back and grinding XP by killing every hostile unit on the first map for no reason whatsoever, making me wonder why I had bothered using stealth and diplomacy to avoid fights in the first place. It gave me flashbacks of playing Ultima 2 and grinding XP by going to a specific town where all of the townsfolk gave 40 XP a pop, killing everyone, and then leaving and returning so that they would respawn.
Yeah, if you're using XP in DnD (or similar), it really needs the DM to award full encounter XP for encounters if the PC's smartly sneak/talk their way through, as well as award XP for purely social encounters (provided there is some kind of challenge/goal involved). Otherwise, if feels like the game just wants you to kill, as you note with BG3.
And yeah, I'm not the most attached to 'equal levels'; some of my favorite games are White Wolf and its like, where there's no levels at all, just freeform spend XP on whatever. But however character advancement is, I strongly prefer players to be in lockstep with one another.