I wouldn't be so sure about keto. Buffet restaurants rely on serving cheap ingredients, which essentially means carbohydrates.
Everyone's first thought about keto is all meat, but vegetables are (or should be) a huge part of it. It's just a cool benefit that a night of brisket doesn't knock you off your diet.
I experimented with a keto diet for a neurological condition. I invested in some blood ketone measuring strips and was surprised how difficult it was to stay in ketosis. Anything more than a medium chicken breast in one meal does it for me. Other metabolisms may be different.
Granted, I haven't been to a buffet in over a decade. They seem to have died out entirely near me.