After living successfully with iPadOS on an iPad Air 2 for two days, I installed iOS 13 on my primary iPhone 8 last night. It's even more stable on the iPhone 8 than on the Air 2. None of the apps I use regularly seem to have any problems, and I have seen only minor bugs, almost all cosmetic. (And some new features, particularly new parts of the Health/Salud app aren't localized yet. Some new text shows up in English along side holdover Spanish text.)
I'm not updating my primary Series 4 Watch until late in the beta cycle, and then probably only if I'm testing something for somebody. I have no interest in dealing with an unreliable Watch.