I have ridden clipless almost exclusively since the late 80s, except for a short time as a courier in a big city and, more recently, off road riding with my new fat bike.
Two things I noticed: there doesn't appear to be a major loss of efficiency using BMX-style flats vs. clipless, and off road riding is much easier with clipless because your feet don't get bounced off the pedals.
Last time I was trail riding I was able to clear almost every obstacle and trail feature on more technical trails, but on faster downhills my feet got bounced off a couple times and made for a few hair-raising moments. I tried clipless on my fat bike but the wide Q-factor put weird pressure on my knees so I reinstalled the flats.