My experience:
I've been postponing updating my phone and watches for months because of this. I was on 17.0.3 on my phone and my watches (AW8 and Ultra2) were on 10.0.something. The other day my phone updated to 17.4 without my permission. I probably accidentally told it to update or something. I was concerned but I had no issues with with battery life on my watches. So yesterday I updated to 17.5 on my phone and the battery on both of my watches went crazy. Both of them started dropping like they were falling off of a cliff. I updated both watches to 10.5, restarted both of them and they've both been fine ever since. I wore the AW8 yesterday and it took over an hour to lose 1%. I ended the day at 66%, which is about normal. This morning, I took the UW2 off of the charger at 6:00. It's 7:40 right now and it is still at 100% (raise to wake, not AOD).
I don't know if I would have had the problems with battery if I'd updated sooner, but 17.5 with non-updated watches was definitely an issue. All is good now though, and I couldn't be happier with it.
I don't know if this helps anyone on the fence about updating. For me, 17.5 and 10.5 seems good for both the phone and the watches.