The combo units work fine. I have been using Garmin plotter/sonar combos for about a decade with no problems.
One issue to consider with the new touch-screen models, and the same applies to all similar devices, is that the screen is very sensitive, and can freeze if it gets wet, so a reboot is required.
I am just about to have a new RIB commissioned, and have considered a touch-screen, but having read the experiences of several of the teams in last year's PBR Round Ireland Challenge, I am going to stick with the traditional type.
Others might have different experiences, but I hope this helps...
Happy when wet!