I've been using mine at the gym for a few days now, and it's not very useful for weight lifting. My Mio Alpha isn't perfect for tracking, but it's much better than the Apple watch. The AW will go for considerable periods where it just says "measuring" and the HR grayed out. It will also on occasion show 60bpm after I just did a bunch of box jumps or dead lifts, which seems... unlikely. Lastly, you're not going to be doing anything with kettle bells in the rack position, as it's even worse if you try to use it on the inside of your wrist (as I can do with my Mio).
I have, however, found it to be very good for the exercises that are built in that I've tried: rowing, running, stair climber. Maybe that's why it doesn't show "weight lifting".