Updating the iPhone and iPad to the beta at WWDC isn’t too difficult to reverse but the watch is totally different. With watchOS if you want to go back a version then you have to send it off to Apple.
This is a process that doesn’t take long (typically around 4 days) but in the current times may take longer. For me I’m using the watch every couple of days so I probably will install it but if you rely on your watch every day then I’d avoid.
Your battery is going to be down and it may lose fitness data, it’s done both of these to me in the past so be warned.
For me it’s an easy choice if you use and rely on your watch every day, as tempting as it may seem – don’t install watchOS 8 at the early beta stage.