my experience with garmin (vivoactive, not fenix) was that they had all kinds of precision and detail, but it also didn't even notice when i got up to pee in the middle of the night - just a straight line through the graphs as though i hadn't woken up at all. that was the end of my trust in their sleep tracking.
Mine reliably notices that I woke up and immediately go under when my wife comes home late in the night, which happens basically every day (because bartender) so I get somewhat statistically significant results in that regard.
The Quantified Scientist does very detailed, measurement-based tests on all sort of smart watches, you might want to see where your device stands in his charts in terms of accuracy.
my garmin was really good at picking up my heart rate. there was one time i had it in a waist pack, and i was wearing a chest strap that was not paired to the garmin, and the garmin managed to pick up a pretty accurate heart rate for most of a bike ride.
