I can't speak for the bigger units, only the little 4"ers that I have on my boat.
I've got a Humminbird Pirahnamax 4 DI on the bow (transducer on the trolling motor), and a Garmin Stryker 4 on the back.
The Garmin is a far nicer, clearer, and easier to use unit.
I do not know if the Humminbird transducer does not like to be mounted on the TM (I've had the Garmin transducer on a TM before and it worked fine), but it has never worked that well. Lots of noise in sonar mode, and the down imaging is next to useless.