Do any of the Rachio approved sensors measure rainfall accumulation?
To the best of my knowledge, rainfall accumulation is only used from weather stations. The sensor that you would have at your house only works as an on/off, water/don’t water switch for the Rachio.
If they cannot differentiate between 1 mm and 50 mm are they still useful?
For me, very much so. My PWS is 2.5 miles, and it can get rain when I don’t and vice verse. I’d put my own weather station in, but I don’t have a good location. So the rain sensor will pick up and shut the system off whenever it rains at my house, no matter what happened at the PWS.