In fairness some bosses are less predictable than players (RNG on boss attacks). But even so, you can learn what to do for every single attack a boss has so that unpredictability becomes unimportant after a certain point. With pvp its usually fairly obvious what most players are doing. Occasionally you will fight someone who can actually trick you, but really everything can be predicted with the right thought process.
Back when i played FPS games. I got to a point where i could predict what players were going to do 95% of the time so it just came down to either me winning out no problems or if they could predict me aswell then it was just who had the better aim.
The balance is that most boss attacks have a tell… in fact I think really all boss attacks have a tell that you can react to. Trying to react to a point blank LB 5 from a warrior is pretty darn difficult compared to dodging a Lupi kick. Also keeping in mind a boss may have 3-4 likely mechanics it will use, while a player may have upwards of 5+ dangerous tells that you should be looking for and reacting accordingly.
I tend to think the biggest difference, say in the case of thieves, is just the choice to spam-to-win. Something that doesn’t happen in PvE, that happens quite often in PvP. 2222222222222 thieves for instance, which can be a perfectly valid strategy against a necro if they are under 50%… Bosses thankfully won’t just stand there and spam the same skill over and over again until you run out of dodges.
I am not sure the AI is perfect… or can be perfected… in fact AI by its very nature isn’t going to be perfect, because if it was, it would use the most dangerous skills when you cannot counter them, which no AI in this game at least does. How many bosses hold their highest damaging AOE till they bait out two dodges? Players do that all the time though.
Killing scores of mobs that can down you instantly, like clearing a full path of any dungeon designed for 5 people is more about kiting, invulnerability mechanics, etc, even though obviously skill is required. The game doesn’t provide you enough avoidance skills to do a 5 man dungeon alone without abusing the limitations of the AI. Mostly the AI melee limitations.