Actually I was talking with a police officer the other day on this very subject. His view was that in general satnav's whether bespoke or on phones are pretty accurate on the mph or kph front and most are pretty up to date on the road mapppings and speed zones, in fact to within 1m. The one aspect when they can struggle is when a vehicle accelerates very quickly in a gps challenged area, as the trigonometry mapping of the position can be temporarily lost and accuracy lost (delay on accurate speed readout). He said that most car speedos were in fact 5-10% inaccurate and were deliberately set to underead speed.
