They aren't but they do have drawbacks. Personally, I don't like SPD for MTBing because your float and release angle are coupled to your tension and this constrains the use of bike-body englishing. Also, earlier and to some extent even current model SPD pedals tend to clog up in heavy amounts of mud. For MTBing (and if you're looking for a walkable alternative to road pedals), I prefer the Time ATAC.