I rode spd on mtn bikes since mid 90s, when i first tried flats recently, I thought they were terrible. I could not keep my feet on the pedals, couldn't spin, couldn't bunny hop. I would hit a bump, my foot would roll the pedal forward and come off. They only lasted a few rides. About a year ago, I tired them again because I had a few bad crashes in tech stuff and some "ride the bike into the ground" wash outs. Over time, I learned how to ride with them, just like I learned how to ride SPDs when they first came out. They feel more stable then the spds, I can use different foot positions. And with heel down technique, I can spin. plus I have the added safety of being able to step off the bike when things go wrong
I think the whole SPDs efficiency is overblown, especially when it comes to mtn biking, you aren't spinning along at a nice smooth cadence.
Flats, fat bike and droppers are all things I said I would never use....then I tried them and saw how great they were