I once had SPD pedals on my mountainbike. They clogged with mud often, and then once on a frigid January day, the mud froze while I was clipped in. I came to a slow, controlled stop on a fire road, but I couldn't get unclipped.

This wouldn't have been too big of a problem, except that I was recovering from a knee dislocation. I happened to fall on that knee when I couldn't get unclipped.

I gave my SPDs away and switched to Eggbeaters. I've never had a problem with mud clogging since. I've got 8000+ miles on Eggbeaters and I've never once had a problem with them (I've had to regrease them once or twice). They work as they should every time. In my experience, they define reliability.