In the current Beta, I have had awful experience with Siri on the watch. It seems fine on the Mac and iPhone, but I get mostly frustration on the Watch.
1) Other folks, and myself have had problems setting Timers or Reminders. This obviously worked pretty well on OS 26.
2) On the watch, Siri constantly asks me to clarify what I said. For example if I say "Trash Bags", Siri will ask "Trash Bags", or "Hash Tags or "Dirt Bags"? So I repeat "Trash Bags". Sir then goes "Trash Bags" or "Dirty Bags". I reply "Trash Bags". It usually then will try a 3rd time, then it gets it right or aborts without another word.
Overall, I am surprised when Siri actually works on OS 27. It's about 50-50 chance for me. I hope Beta 5 fixes this. The Betas on the other devices have worked pretty well for me, even Siri.
At this point, I wouldn't install Watch OS 27 if you use Siri.