I would argue that Diplo victory has one major flaw:
its time cost. As far as I know, you can't even earn any victory points in the first couple eras. All you can do is scrounge for envoys to claim suzerainty and accumulate Diplo Favor.
To earn points, you either have to vote correctly at time-gated World Congress sessions, unlock a handful of specific Wonders (which take time to build!) throughout separate eras (with the Statue of Liberty coming as late as the Industrial Era), or wait for certain competitions/emergencies to appear on their own.
Now, building Wonders and protecting suzerainty and clicking next turn is far from difficult. However, you can only speed up the process so much. Sure, you can try and rack up
Science and
Culture to bolster your way through the trees and unlock key Wonders faster (as well as unlocking the helpful diplo-focused Future era civics). You can try and boost your
Production to build your Wonders faster, or stockpile
Faith and
Gold for certain Great People (huh, that's a lot of different colors...). But all of that still can't truly overcome the waiting. Wait. Wait. Waiting...
Now, disclaimer:
Diplo is my least sought after victory. Frankly, I find it kind of boring. It's like turning the late game "next-turn" marathon into the entire game.
I don't have much experience with it, so this is mostly theoretical talk here based on my hours spent in the rest of the game. Regardless, I would disagree that Diplo Victory is overpowered or much easier to achieve than the other victory conditions. While it's free of a lot of conflict and its individual steps are pretty easy, it's held back by its glacial pace.
it can still be fun though