I run SPDs on MTBs as well as the dedicated trainer bike. SPD-SL on the road bikes.
SPD-SL feels a bit more secure, but SPD feels fine too with MTB shoes. Unlike road shoes, where only the cleat is in contact with the pedals, MTB shoes have contact points at the cleat as well as the sides of the shoe. The grip is even more pronounced if you run “trail” or DH pedals with bigger cages. My highest 1-3 second power numbers are consistently recorded on the MTB, so it’s clear that the SPDs are sufficiently secure.
I don’t know if SPDs will fix your hotspots. They might. IME it’s more about shoe fit and making sure they aren’t too tight. I had horrible hotspots with Specialized shoes. They just didn’t fit my oddly shaped feet (paddles). Tried many things, including insoles, Speedplays, pedal extenders, cleat shims, etc. Finally bought Sidis and the problem went away. Now I own 6 pairs.