I started writing this post with incoherent ramblings so I deleted it and started again.
I use my AW S7 for tracking running and hiking workouts mostly so my domain is mostly related to health and GPX tracing. I haven't really had any problems with the AW S7 or my SE on that front. They have been extremely reliable and last a full day without a problem. Mostly used with the Compass app, OS Locate and usual Workouts app. If I do a multi-day trek I'm already dragging a large Anker powerbank with me so throwing the charging puck that came with it in my backpack is no big deal. I know a lot of people with SE, S6 and S7 who come out on our group walks and they never have any problems or complaints about theirs either.
There are three minor annoyances which I'd like to outline though:
1. If it gets damp, occasionally it'll go on the fritz and stop activity tracking. Wiping it down fixes that.
2. It's not that easy to extract the GPX data for a workout hiking route from the health app. You have to download a 3rd party app to do it.
3. It thinks I'm insane after walking for 5 hours solid (I can do 12 hours and 50km) and keeps asking me to stop activity tracking.
With respect to fitness, running in particular, Apple wins for me again because it works with Apple Music and my AirPods. I go out with the S7 and the AirPods and that's it. I don't want my phone smacking my ass or strapped to my arm while I'm running.
Conversely, our few Garmin (Instinct/Fenix) users seem to have endless problems with theirs. On two separate occasions now I've had to lend someone my backup eTrex 10 because their watch has bricked or the GPS has given up. Also they seem to crash occasionally or freeze which you don't notice. Also, and I think this is Android more than Garmin, getting them to sync post-hike appears to require at least two pints of beer and a bunch of swearing ?
No one uses fitbits that I know. They are outcasts
One point to note though is that at least 50% of the people who have Apple Watches that I know got them free with their additional corporate private healthcare insurance as a benefit.