I have had a few false positives while washing dishes, but they always cancel automatically. It doesn't bother me. I suspect my dishwashing technique doesn't produce the kind of sustained hand motion the app is looking for.
Since I wear my Watch on my non-dominant hand, that hand is generally used for holding/steadying dishes while my dominant hand is vigorously washing. Wearing a Watch on your dominant hand could conceivably make a big difference in this regard.
Overall, I find this to be a cute feature, but not earth-shattering. I was already used to counting to 20, so learning to trust my Watch to start counting automatically took a small mental effort. The taptic is a bit weak for my taste. I think that all the tactile feedback from hand washing pushes that taptic into the sensory background.