I haven't yet been to Boma's but reading so many reviews, it seems that it is not the sort of place that is going to let you just have piles of dirty dishes at your table and a server who just comes out once or twice with clean plates.
I really don't like buffets myself, but went to Golden Corral for the first time in five years about a month ago. Our server worked his rear off. He had a lot of table to take care of and as I thought about it, the only thing a typical server does that this guy isn't doing is write down the order and carry it out from the kitchen. This man didn't have to get our food, but he had to take away dishes and bring more. He had refill our drinks. He brought me coffee. I looked at the probably 15 tables this guy was working (he was moving the whole time, non stop) and thought that he isn't going to make hardly anything in tips because it is a "buffet". I left him a nice tip and certainly would do the same for any other buffet server that worked like that.
I could be wrong, but I suspect that is what Boma's buffet treatment is like. 18% or more is acceptable IMO, if that is the case. (Anyone feel free to correct me if I am wrong about Boma's service).
Now, if a buffet server did nothing but hand me clean plates, letting the dirty pile up and I had to get my own refills, etc. I think ten percent is fair.