In my experiences it seems accurate enough, it just takes a while for it to reach the correct level. Especially in intervals.
For example a 3min interval:
Power is instant, chest heart rate stabilizes in around 30seconds. Wrist based takes longer.
Those are based on my personal experiences.
In normal everyday use it seems ok. Tested my Fenix 5 vs a pulseoximetry device, heartrate was the same on both.