I personally use my Garmin watch to record the data from my mountain bike. Having an actual bike computer while mountain biking wouldn’t be useful during the ride on any of the trails I ride. Simply to tight and turny to be staring at a screen.
But post ride, it’s great to compare efforts in strava. I’ve got a speed sensor and cadence sensor on the mountain bike and will eventually get a power meter because I’m interested to see what effort it actually is taking.