Sorry, I was wrong, I corrected my post. Accelerometer in the phone doesn't use magnetic fields to detect motion. I confused the function with motion capture sensors which contain a combination of other types of sensors.

Anyway, there is a chance that the magnetless sensors sometimes gives occasional false readings from bumpy roads. You could look at threads and blogs reviewing them. Although overall, I don't think the errors would significantly change the total distance and average speed. You might get some spikes though. I decided to stick to the old CooSpo model that uses a magnet for a more conventional and less error-prone mechanism, and less likelihood of false activation when not riding the bike.

One benefit of the magnetless sensors is the ability to quickly remove and reinstall them with the rubber band without needing to adjust a magnet. Some reviews say the rubber band breaks and they lose the sensor, while others say it's fine. You could use zip ties instead, but then it's no longer easily removable.