Downhill full face helmets for white water?
So I would like a full face helmet. I can't get any uglier and remain employable, so I gotta keep this mug looking at least the way it does now! I've also got a large head, and I've tried ALL of the full face whitewater helmets out, even with the padding removed, in all of the large sizes. ALL of them give me headaches as they are just barely too small.
I considered MX helmets (I have a bunch) but the story goes they're too heavy, which would make sense as messing with your roll. I also imagine they're built for pretty high speed impacts, and probably wouldn't do much for the average river crash speed.
So on that note, I noticed full face mountain bike helmets are not only light, but REALLY affordable! All of them are under $100, have way cooler designs, etc etc.
So... is there any particular reasons not to use one?