I ride both – SPDs on my singlespeed, rigid mtb and road bike, flats on full sus and fatty.
These days I prefer flats for mucking about but I do feel spds are more efficient for pedaling (happy to admit this may be placebo) and definitely prefer them for SS when gurning up a steep hill at max heart rate.
Flats can help improve technique, both by forcing you to work out how to lift the wheels without pulling up and by giving more confidence when manualling etc. However, ehen I first tried them I noticed I was dabbing at points where I’d have just pushed on through when clipped in, so it can maybe go both ways.
I get more pedal strikes with flats and I’ve had to get a boot dryer because Five tens are made of sponge.