Protomech does a great job of outlining the history and pros and cons.
Skadamo runs both EMF and BF, with me as a co-administrator on BF.
I keep up with the BF posts by subscribing (clicking the "notify" button) to almost every new post or by replying to a post, which automatically subscribes me to that thread. I do the same thing on EMF, but only get "new post" notifications on certain subforums.
Richard230 is great because he posts news on all three of the forums you mentioned. I really appreciate him for doing that.
My experience with forums (inside and outside the EV world) is that they all "fragment." A big forum breaks the information up into subforums. Those subforums often divide like cells when new subjects come up and a need to parse things into sub sub forums becomes apparent. Then, someone comes along and starts a whole new forum on an issue that may have started on a thread deep in a subforum. And on and on it goes.
Combining these back into one point of origin, a super-forum, so-to-speak, seems to go against the natural flow of forums I outlined above. We're probably not going to do it with BF and EMF, but perhaps skadamo can chime in with his opinion.
And, while I started on elmoto way back when, I rarely go there today. This has less to do with the administrator - who is a great guy - then it has to do with some of the people on the forum and the culture that goes along with it.