[pfranz]

Missing ANT+ was mentioned above. I was recently looking at the Apple Watch and rowing machines. Ignoring that a wrist-based heart monitor isn't great for things like boxing or rowing, you can't really get the heart rate monitor to connect to gym equipment. Instead of ANT+ or Bluetooth LE Apple did release GymKit in 2017--but very few things support it. The "rowing" exercise doesn't measure things rowers find useful (it's basically a generic cardio workout). When using a separate heart rate monitor and syncing your workout with Apple Health you can get double measurements heart rate (the AW constant heart rate and the strap during the workout).

The fitness industry has a pretty well-developed ecosystem (I grabbed a random 5 year old heart rate chest strap and it paired immediately to the rowing machine). Garmin has seemed to have embraced that for at least a decade. It can definitely use improvement, but Apple has its own thing and it'll give you grief if they didn't have your use-case in mind.

To be fair, others like Fitbit has similar issues. With them you can't even grab detailed heart-rate data.