See, I don't think you can generalize like that. We have lots of ethnic restaurants where I live. Now, I have never visited those parts of the world, so you may be right there (I do have family who are Indian and they've made some wonderful ethnic dishes that are much better than any Indian restaurant's offerings). I wasn't expecting Boma to be like my local Ethiopian, Indian and Middle Eastern restaurants, and it wasn't. But I also wasn't expecting an American-style meat-and-potatoes kind of buffet, and it wasn't that either. It's in-between, just "Boma" by itself. I'm sure Jiko is nicer (being a signature restaurant and all that), but as a vegetarian I appreciate the wider variety of food choices at Boma. For me, it lends itself to repeat visits whereas at Jiko I have only one entree choice.
I think if someone's not sure if they're going to like any restaurant, they won't know until they go and actually eat the food. Nobody can predict how someone else's tastebuds will react.