I am in permanent AF - a largely asymptomatic condition for me. Recently I went into an emergency department in Australia on a non-AF related issue. While there I was hooked up to the hospital heart rate monitor, the type they clip to your finger. It recorded a very high heart-rate, then plunged suddenly and set off the alarm at intervals as its programming tried to figure out what was happenning. Eventually the nurse took my pulse the old-fashioned way using her fingers, and the value was pretty close to what the Fitbit Charge HR on my wrist said. She was quite an experienced nurse and said that the monitor had trouble with AF patients.
I was used to taking my heart-rate long before I had AF using the old heart rate monitor chest strap, which was useful when I entered my AF, but did over-record during exercise. My Fitbit seems pretty accurate at rest but, as with many non-AF users, it records a slightly too low value when exercising.
The reason the Fitbit (and maybe other wrist wearable devices) work for AF users seems to rely on its programming which has a distinct lag in recording trends in the rate. So while a super accurate device reads a couple of seconds of beats together as a sudden rise in heart-rate, the Fitbit ignores it assuming that the less reliable method it uses (of a pulsed light) has made an error, and averages out over a much longer period.
So in a way the same thing that makes these wrist wearable devices less suitable for non-AF athletes actually makes it more suitable for those experiencing AF.