Descent Mk2 has built in activity profiles for just about every sport you can imagine. And for fitness analytics (including sleep tracking and calorie counting), Garmin Connect is generally superior to anything in the Apple ecosystem. But it can't download medical records from other providers like Apple.Probably the ability to interact with the rest of the Apple ecosystem. It's hard for me to imagine a feature that would tempt me to forgo my Watch's interaction with my iPhone and associated software, especially the health and fitness apps.
One other limitation for iPhone users is that third-party smart watches can't be used to reply to text messages (only view them). In anti-competitive and anti-consumer move, Apple locked down that API so it can only be used by their own watch.