Clipless pedals only unclip when you turn your foot sideways and mtb (aka spd) pedals are no more likely to unclip than road pedals. Most pedals have an adjustment that allows the unclipping force to be set high enough to prevent unwanted unclipping. I've been using the cheap Shimano M520 SPD pedals with the SH51 cleats for years w/o any problems.
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