Besides limited variety, the other problem is you need to choose very conservatively. Lots of options have animal-derived ingredients and the staff are either clueless or there exists commonly a significant language barrier.
I will say it's an improvement on most restaurants simply because you can actually find naturally vegan options that aren't a sad salad but its tough otherwise since every place is different and uses wildly different recipes for a given dish.
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.
Buffets were really a big part of Sabbath afternoons for many of my childhood years, and we knew many other SDAs for whom it was the same. The local SDA hospital still has a Hometown Buffet nearby that hasn’t been closed yet.
(We were non-strict enough to allow paying for something on Sabbath — though that would have crossed the line for many people we knew.)