I rely on my Apple Watch to tell me when I am in Afib and it has been validated by 12-lead ECGs on several occasions (I am speaking about my own experience, not research). I have learned that it is the p wave on the ECG that is relevant to Afib and the ECGs show that very clearly. It is not intended to catch any other abnormal rhythms. I was able to talk with someone at Apple Support about the algorithm the watch uses to report Afib and was told it has to pick up 5 consecutive Afib readings before it tells me I am in Afib, but the good news is that it picks them up automatically without any awareness or action on my part, unlike the Kardia. I used the Kardia device before I bought the watch and always wondered whether I was just increasing my anxiety because of the effort required.