Sorry to hear of your disappointment in the nav system
With well over 100,000 miles on two 2006 DTS's, I am pretty happy with mine.
I don't seem to have all the trouble that you do. (For which I am very grateful)
We travel a lot and it works pretty darn good...but I do not have a Garmin to compare it to, I have never needed one...yet.
Maybe the DTS and STS use a different map system or display system, but mine shows the NAMES on the POI's when I zoom in. If I am zoomed OUT... I have to touch each one to get the name.
I sometimes just push the OnStar button...tell the nice lady what I am looking for and she tells me how to get there.
Saves a lot of frustration.
Texas Jim