[Someguy]

A unit of burna boyz in a battlewagon can actually be devastating. The thing is, when putting the template down from a transport, you can count all the guys as firing from the same place. So you put one flamer template down, say on 5 models, but if you have 10 guys firing that means 50 hits. The target unit often isn't there any more.

You don't want all that many burnas though. They don't score and they are useless against people in transports. They can be a useful part of an army but I don't think they make an army on their own.