I would disagree.
I've got 4 vehicles with built in Nav systems (2 VWs + 2 Audis). All 4 are vastly superior to my Garmin. And quite honestly all 3 are superior to every phone-based system I've used.
The best of them have dual screens, one in the center dash and another in front of the driver, which stand alone and phone systems. The best of them show multiple roads at intersections to make sure you turn at the correct point. The best of them show complete roundabouts with correct directions. They mute the radio during instructions, which stand alone and phone systems cannot do. 2 of mine update the maps automatically via satellite links.
Its impossible for a phone system or a stand alone Garmin to do what the better built in Nav Systems can do.
A marginal/average built in Nav System is no better/worse than a stand alone/phone system on most things.
A bad Nav System is miserable.
For this trip we were using both iPhone and Ford systems.
Actually, if I use the Google maps app and leave it bluetooth connected, it will mute my radio to announce things (if I let it).
One thing my Garmin has that my iPhone does not: the Garmin works even when there is no cell service. There are plenty of spots right here in Indiana that my cell map app is completely worthless.
Heck. Made it across the lower 48 several times in a semi using one of these....
I made a few wrong turns but always found where I needed to go.
If I need the newer technology, Google maps suffices.
I don't have much sat nav experience, but I rented a Chrysler 300C when I was in Los Angeles last fall. The built-in nav on that thing was spectacular. It was dead-nuts accurate. One thing I really appreciated (not knowing L.A.) was that it told me early what lane i would have to be in for the next exit or turn and displayed a graphic that made it unmistakable. In fact, the whole car impressed me...which surprised me.
I rented a different care with a stand-alone system my second trip to L.A. last year and it sucked. Same on the Ford Fusion I rented in Rochester N.Y., but luckily Rochester isn't too difficult to get around.
FORD, on the other hand, apparently offers the opposite end of the spectrum. I think it might be the rectum of the Sat Nav systems.