No problems at all running that even on hard terrain. Yes the bottom bracket will be 12mm lower, and you may have to ride a bit stiffer on the suspension for example if you're striking the ground all the time.
I've been running a 26" fork, 26" wheels on a 650b frame (with already a big -12mm BB drop ...), and it's fine. Yes I've struck the pedals a few times, but on a frame with a "regular" BB drop like -5mm, it should be a lot better.
I'll probably switch to full 650b sometime, as I have the wheels already, but I can't find a 650b fork that would suit me yet.