My car has built in sat nav but I tend to use Google Maps on my (work) iPhone, linked through to the car display via Apple Play. So far so good.
Occasionaly the Google Maps display glitches and has my position out by a couple of miles. It did this a few weeks ago when I was driving home and kept trying to snap me to the roads nearest to where it thought I was.
Extra fun yesterday, driving through West Sussex, Google Maps up on the display but navigation mode not running. It had my position wrong, bumping across fields. For added fun, I brought the in car navigation up. The car has two displays, one middle of the dash, which is where Apple Play puts the Google Maps display, and one infront of the steering wheel, where the speedo etc are situated, so both were visible at the same time. The weird thing was both the Google Maps display and the in car navigation had the same wrong position. The direction of travel was out too, by approximately 180 degrees.
After 20 minutes or so Google maps showed the correct position while the in car system still had me two miles adrift.