because downhill is a fraction of peoples rides and an equally insignificant fraction of their attentiveness to power measurements.

over a typical ride distance and profile the measurements converge.

all the 50-150km group rides ive been see consistent time, speed, and profile reading, whether reported from garmin, android, or iphone. and being that those (w system weight) being the key factor in power reading (w assumed average CD/WR & RR) , its decently good when compared to power meters*...

...which also arent infallible....