I don't have that problem with the device that's being reference, it puts out a strong FM signal that overrides any weaker FM signal; clearly a strong FM signal might be a problem.
It's a nice unit as I have one too and it will auto search for quiet FM stations, so tweaking it to another station is a piece of cake if you need to.
But as another poster wrote, it is a bit weak on the bass, not sure if that's the phone of the BT transmitter, although I think that might be Pandora's fault as music files on my memory card do sound better. For the most part I like it and it is definitely one of the better FM transmitters I've come across.