You'll lose the depth and space that stereo gives, of course, and the overall sound will be much less realistic.
Any DSP effects that use stereo effectively, will be less effective. Sorry, that's a triple pun, but I can't think of other words right now. I haven't closely examined the rotary DSP effects, but if they use side to side panning to help recreate the Leslie speaker sound in stereo, they'll usually sound hideous in mono, with the tremolo being at double speed with a non-tremolo sound in the centre. Same thing would apply to chorus and stereo delays.
So if you have a stereo amp, don't feed it with a mono signal from the keyboard. If you have a mono amp, then you have to feed from the mono output and accept the drop in quality.