RPGMemes

A party can definitely be too powerful for the narrative you want to tell. If you've got a specific story with specific challenges in mind, then players can have abilities that can render those challenges moot.

Using Pathfinder as an example, but this would apply to several related systems too: You have some kind of ravine or other traversal challenge. Prior to gaining access to fly, the players need to use their skills to find whatever solution you've seeded into the world or come up with some creative solution you haven't thought of yet. After gaining access to fly, they can simply fly across, nullifying the challenge.

All this is to say that in certain systems, it's not simply the numbers but the nature of challenges that change as you gain power. A high-level party tells different stories than low-level parties (this isn't an accident, it's a feature), and if that's not the kind of story the GM wants to tell, it can be a problem. Arguably, however, that's something the GM should have been aware of when going into a system like DnD or Pathfinder and been prepared to raise the stakes to entirely different kinds of challenges.