Actually, the opposite is the case. The sensor is, according to most tests that you can find, shockingly precise. Fitness trackers (not watches) like the Fitbit or Garmin models you listed have much smaller and thus automatically less precise sensors. I've seen the comparison with my Fitbit tracker before and the Watch4 now. The Watch4 outperforms it drastically.
As mentioned before, the limits for both too low and too high heart rates for a certain period of time (I think 10 minutes below or above the set value) can be set in the Samsung Health app on the watch.
