Do you mean that only a bad GM would make a bard player face a high level opponent?
I mean only a bad GM would make an encounter without accounting for if the player can beat it.
Because they both can only if the GM isn't an **** or horrible.
WHAT CHALLENGE?! You haven't even spoken of a challenge other than some theorized "high level campaign." If it's not designed to be beaten by the class facing it, chances are that the class may not be able to beat it. Doesn't matter if it's a Fighter, Wizard, or a Bard. You throw a Fighter into a high-level challenge that consists of tons of traps, he's going to have a problem with it. If your Wizard has to fight in a Wild Magic area, he might have some troubles. Same as if you throw a Bard into a ring with a raging berserker, he may theoretically also have trouble. However, at the same time, he does have spells that if chosen appropriately can help him overcome it.