S3 on 6.2.8, iPhone SE 2020 on iOS 14 Open Beta. Watch runs great, no adverse affects. In fact, since updating to 6.2.8, I have noticed my battery draining issues diminish.
Have two S5 on 6.2.8 and iPhone 11 on iOS PB14 and no "issues". What I have noticed - and don't remember if this was before I put the Beta 14 on my iPhone - is when I am out with the Cellular Series 5 and not the iPhone, I am getting Mail notifications from non-VIP people, which I didn't get before. Not sure if it is related to the Beta iOS 14 or not. Did change the notifications settings for the Cellular AW from Mirror my iPhone to Custom - read in another thread that that solved the issue for someone.