It depends on the sport. It works well on cycling/spinning but it works very bad while running or swimming.
I think it is related to the movement or maybe because you are sweating more. I don’t know, I practice triathlon and only have good measures while cycling.
Not to mention that when I wear my chest strap the number of rpms decreases always around 20. The difference is so big between the watch and the strap.
I had a S0 in the past and have a S3 nowadays. Both the same. I don’t believe any HRM measures from the watch, just use them as an “indicator” or when I forgot my chest strap.
BR!