Zeitgeist 4e is generally on the very easy side, and can be completely stomped by any group with even a modicum of mid-op characters. Not even high-op, but mid-op. Our group played through books #1 through #8 with three mid-op PCs, and books #9 through #13 with two mid-OP PCs. Despite the DM actually beefing up encounters, we have still been completely crushing them, especially at the epic tier.
The only real difficulties stem from monsters with inexplicably potent one-off mechanics, like Reed Macbannin's curse of the writhing cauldron, or Lya Jierre's sunder, or the various forms of antimagic.
Zeitgeist for Pathfinder 1e is even easier. The monsters and NPCs there are very weak.