You probably know this, but for anyone else reading - I have an Apple watch that has the EKG. It is a two-contact EKG, so it's limited in what it can see. It also requires you to actively look at it (you have to touch the crown while it's reading).
It did successfully diagnose a bout of bigeminy that I had, but unfortunately my cardiologist didn't think it was important, and I ended up having a heart attack anyway. But I lived, and now I have a new cardiologist, so there's that.