I'll flat out say that sometimes that's, in practice, a dodge. There's two reasons it can be:
1. Its easy given the time frames and such of a game session for that "talk about it later" to just never happen. Even if that doesn't happen every time it requires a very persistent player to make sure that their concerns are always at least heard out.
2. If you have a GM unwilling to roll things back, addressing a problem later is sometimes useless. Its hard for me to believe most GMs don't, on some level, know that either. While an overly dramatic comparison, "justice delayed is justice denied".
This doesn't mean that every case is important enough to justify taking the time to thrash it out, but its a very uneven process powerwise, since in practice it means that if the player thinks it is, and the GM thinks it isn't, there's a strong tendency for the latter to get to win.
The problem is, some of these things aren't the sort of thing that will come up in campaign setup discussion. They can be as simple as the GM being casual about things that a player thinks are important, and they may not find that out until well into the game when it keeps coming up. This can cover all sorts of ground from consistency of decision making (and transparency for reasons for the decisions) to how clear communication on things like difficulty is expected to be.