although Garmin has made strides with the latest generation optical heart rate unit in their newest watches like the Fenix, Epix, 955, 255 etc. it still isn't as good as Apple's in the S7. All that said while Apple is the best to do it at the wrist and does a reasonable job with HIIT and lifting, it is still hardly perfect for those categories. An optical band from the likes of Polar or Scosche worn on the bicep / upper arm does a much better job or a regular HRM strap worn on the chest is the best of all (as long as it is warm and your skin has some moisture or gel.) Apple Watches work with all options of course.
I used to have the Scosche arm band that was pretty decent at HIIT, it sounds like it's worth getting a secondary band. Although in all honesty I don't really pay much attention to my HR and zones, I'm more old school and just use RPE. Especially for HIIT which is really all out, it doesn't necessarily matter where your HR is anyway. I just gripe about it because it's annoying to wait 10-20 seconds for a zone change, but it won't change my workout.