1. Depending on front location, they may interfere with the headlights when driving at night.
2. Hanging them on the front will allow them to pick up bugs and road debris you now see on your grill. Covering them will cause airflow issues to your radiator.
3. They can be mounted on the back with a swing out carrier to easily gain access to the TC.
4. You can also look at ladder attached bike mounts.
5. Even though your bikes will be trailing you if you mount them on the rear, the vortex will still ensure that they are dirty when you get to your destination.
If mounting externally I think the rear works better, but we usually have an enclosed garage behind us that keeps the bikes clean and dry.