IIRC, the Bumbling Brothers essentially concluded that they weren't quite sure why they worked, but were sure they could reliably "glide" as an incredibly easy qualifier in NAR competitions. Mark Fisher subsequently advanced their findings at N46 by getting into why they worked, then tried to propose a couple design attributes that could lead to truly competitive performance for a backslider. His flight tests proved out about half his theories. I'm not sure anyone's done any serious research into it since then (other than at least one I know of that generally argued they should not count as gliders/generate no lift, which has previously been shown to be just plain wrong).

I wound up flying in one meet last year that included a D-RG event, had planned on putting up a nice slide/flop wing but totally trashed the prototype a week before, so decided to try to come up with a semi optimized backslider to fly on 24mm motors. It was a little too windy for my comfort, I had to add some weight for better stability, but it wound up doing respectably, gliding for just under a minute per flight. The biggest problem was just plain seeing it/tracking it (tiny needle in an overcast sky).

Backsliders can certainly come across as almost cheating your way into a decent flight compared to building a nice glider, and I definitely prefer more conventional gliders, but I have to admit, especially when I got into the follow-on research about why they work and how to improve their performance, it really struck me as a fascinating design problem/solution.