Oh, you are so correct about the accuracy of the watch and Afib.
I speak from experience as on June 29 my AW5 warned of an irregular heart rate and Afib. Alarmed, I drove to the local ER where I spent hours hooked up to an EKG, had a chest x-ray, then blood work done. ER report stated that while I was there for possible "palpitations and atrial arrhythmia", they found no abnormalities and suggested I see a cardiologist. I did and gave her the print-out of the Apple ECG showing Afib. That led the doctor to order a slew of tests, including a cardiolite stress test (that was fun

) more EKGs, an echocardiogram, followed by 30 days wearing a Holter monitor. I'm fortunate to have good medical insurance to cover the thousands of dollars these tests cost.
The bottom line after all these tests: No Afib was detected; the prevailing rhythm was normal sinus rhythm.
I'll now always question the accuracy of my watch, not only for heart rate but fall detection as well. Today as I was walking out of the grocery store with bags in each hand, my watch "detected" I had fallen. This is the third time in 60 days my AW5 fall detection went off and the third time it was wrong.