I think a themed CYOR is much harder to balance and play. Take KY's flavor as mechagodzilla, it's an anti-Kaiju mech. The rest of us are Kaiju. There's going to be some kind of bias against him in the first place if he does a reveal versus someone like Louis revealing their character.
In an "anything goes" CYOR you have none of that and there's far more room for creativity. I'm guilty of this myself, but CYOR role abilities should also try to be more creative than just a reflavored role block (I think Cat5's role block item is very creative btw). I remember on a mafia forum over a decade ago I picked "co-mod" for my role and had a lot of fun with the unique goals and abilities I had. Even after all this time I still remember it. I don't remember the random CYORs where I get a one use day vig because I picked a superhero.
However, running those kinds of CYORs requires more players to signup than we've had in the typical CYOR or mafia game.