I have an M2P and an Apple Watch I do usually take mine off before flying. I have left it on and flown without any issues, the issue I occasionally have had is when turning on the ac. I have a magnetic watch band and it may interfere with the compass causing it to require calibration, and it...
A key point is that the 400’ FAA limit is AGL not above launch point. If you launch and fly to a 100’ ridge you can legally fly 400’ above that ridge or 500’ above launch point when above ridge.