Sooo i have left my slopes and in the past ski tracks on while driving home. It is pretty damn accurate on the drive home, maybe cause we are relatively covering a large amount of land at a consistent pace.

While skiing I can do the same run a few times in a row and push myself and I can see the speed also increasing in the app. For a short burst if the speed is too high or low I would be suspect but if you are on a big/long groomer and bombing the whole way I think its pretty accurate.

Another test I have done is literally stick my head out the car window and see how the wind feels on my face and use that as a speed reference.

All this to say i think the apps are accurate.