For Chaos games, assuming leavers are random (not quite true), game balance should not be affected. The effect is the same as have a game with fewer players. Even if the roles are unique, there is no practical difference between a role that was not picked and a role that suicides. It feels different, but the odds are the same. Similarly, if you held a raffle and randomly picked some losers before picking a winner, it would feel like those people were cheated, but picking WINNER, LOSER, LOSER gives the same odds as LOSER, LOSER, WINNER, or picking 3 and then picking which of those 3 is WINNER.
Quitting in Chaos games looks pretty close to random. I have seen just about every role quit, from Mayor to Werewolf. Some people hate town, some people hate neutrals, or they hate particular roles like Medium or Disguiser. The only real worry is snowballing suicides, say if one Mafia quits so another quits so… The same can happen when Town loses a few important unique roles early.
But still, you’re a jerk if you force 14 people who signed up for a 15-player game to play a 14-player game. 1 isn’t a big deal, but it feels like it is rarely 1, so those 3 jerks just forced a 12-player game on everyone else.
In the standard game, leavers are less innocuous, and therefore more problematic. Every role is random in all/any. Roles are added in a particular order in the standard game, so a planned 12-player game is not just a 15-player game with 3 random roles removed. Also the roles are almost entirely fixed, so one missing role gives a lot more information than it would in Chaos.