I think there are challenges in getting an accurate reading at the wrist and it took smart watches several iterations and specific sensor designs to arrive at suitable accuracy. What that is i dont know but it seems the further away from the heart the more difficult it is.
Like other gear, it just disappears out on the trail. I'd feel weird with a helmet, knee pads and chest protector sitting at my desk but i dont think anything about it when at speed at the bike park.