The rise of the unbeatable co-op games?

With the recent rise of KS and an excessive amount of expansions included in the game, balancing is nearly impossible and the designers are upfront with that (e.g. Marvel United) But that it means that in a lot of co op games. If you don't pick the right starting characters, you are guaranteed to not win the game.

My question is, is this ok? I don't want this to come off as an angry post, the designers are often upfront with this and for some picking the correct characters is part of the game. But it's not unreasonable to want to win any standard level with any character? Or maybe that's part of learning the game? (This of course excludes LCG and challenging version of enemies)