I have a few Garmin GPS units and even tho I get the updates as soon as they come out, that does not mean that they are all going to be perfect. There is one RV park that we go to almost every year and for the past several years it has shown it on the opposite side of the road and about a block away from where it is actually located. Our own house shows up as three houses down on everyone's GPS that has tried to find us. As mentioned, a GPS is a good back-up for an up to date road map and for your own eyes.
Only once has my GPS taken me down a road where I got in to trouble. Turns out it wanted to take me over a narrow bridge with a 5 ton load limit. I didn't want to test the bridge since I was way over 5 tons and just turned around and went back until I could loop over to my destination.
From what I understand there are only two sources of map data and TomTom and Garmin each use a different database. For that reason my wife uses the TomTom as a back-up to the Garmin that I use. Between the two of them, we eventually get to our destination but they don't always agree on the route.