It’s subjective; if a player tells you a game is unbalanced they’re saying it feels unfair.
Ok. this is impossible to calculate, but you are probably correct. This gets to my point though, a game can feel very unfair or biased just based on the random dice rolls in play and equal abilities will not remedy this.
Go look at the BG3 forums on Steam and read about all the people complaining about the RNG. It is safe to say the game felt unfair (and unbalanced) to those people even though they were all playing with the same point buy system.
Just s guess but I’d suspect you’d get experienced players complaining at somewhere around 25% difference in overall output. (Assuming it wasn’t just a case of the d20’s being crazy) Probably more if the outputs aren’t directly comparable.
Well, whether it is correct or not that is a number we can work with. If we are using damage as a metric, and you are looking at 25% difference, the vast majority of sessions will be balanced even with characters in the 95th percentile and the 5th percentile.
This underscores my point. I can go into a game with my 14 Strength Barbarian and have another player at the table have a 20 Strength and the session will usually end up "balanced" by this definition.

