I finished an incredibly unexciting diplomatic victory game on Hard because I discovered after exploring the map that every empire was Kingdom!

They were more than happy to form treaties with me and eventually, when I was ready to win the game, an Alliance Victory was simply a matter of clicking a few times on the diplo screen and offering a tiny good bribe.

Unsatisfying ending, but the bigger problem there was the multiple Kingdom AI.

Certainly, you can configure the AI present in each game (and I'll be doing so from now on), but I think by default it should include a roughly even mix of Kingdom and Empire players when you create a new game, to guarantee a certain level of strife.