Aside from all the issues posted already, the big issue for me is that the in car GPS is stuck in the car. The sort of times when I really need navigation assistance is when I am working in an unfamiliar location. Moving between client sites for example - I can be in the office, ask someone where the next meeting is, and get it into Waze there and then. One a number of occasions I've said to someone "is this it?" and showed them the screen only for them to point out that I have entered the site on the wrong side of town. If I was doing this in the car (on my own), I'd end up at the wrong place.
At the other end (and I accept that is is probably more of a European problem than a US one), it is pretty common to be parking some distance from where you actually want to be. With the phone, this is no problem, I walk with it, and my robot overlord tells me to "turn left at the end of the road". In car GPS - not so good.
I also end up driving multiple cars - hire cars, my own car, my wife's car - I don't have the time or the enthusiasm to learn how to drive the complex & infuriating in car Nav systems, the phone does it fine.