Based on personal experiences with using Garmin auto GPS units while motorcycling:
1) The biggest issue I've found is vibration. The power adapter's USB connector to the GPS unit, and/or the power adapter's cigarette lighter connector tends to loosen and even get disconnected. The soldered connector inside the GPS unit also will eventually be broken causing intermittent contacts--at best, charging issues, and at worst, slightest bumps on the road cause the GPS unit to power OFF and reboot randomly. BTDT... a real PITA. Secured the entire power adapter cable--both ends--tightly with zip-ties so that vibration effects are minimized.
2) Heat shouldn't be an issue when using on a moving motorcycle, there should be constant air flow cooling the GPS unit--even when mounted behind a windscreen. I've never encountered any problem while using on a motorcycle but I've actaully seen heat problems when GPS unit is used inside an auto, mounted on the windshield atop of the dashboard--permanent white lines begin to form on the touchscreen.
Your situation may be different, but this has been my experience, FYI.