Some will say that there is less power transfer, but compatibility is not an issue at all. I love Mtn pedals and shoes. I just might need to walk one day, and I can't seem to give that idea up enough to switch to a full road shoe/pedal setup. LOTS of people use this, just not the pros. Lately, I have been into road style touring shoes with MTB pedals, but they are few and far between. My specialized touring shoes look just like a road shoe, but they have a walkable rubber tread across the whole tread. Shimano makes a similar shoe as well. They are a best of both worlds for me. You have to be pretty strong before stiffness becomes an issue in shoes, and a solid MTB shoe is as good or better than a weak road shoe. However, most shoes that don't flex like your running shoes will serve most well.