You can get some pretty fast support vehicles (I mean, 9/14 is no snail!), but vehicles tend to be slow in general, and support vehicles often don't need nearly as much speed-on-demand as their combat vehicle brethren.
One thing to note are small support vehicles, which you can actually get away with some pretty bizarre stuff with. I remember I once made a jetbike with a single auto-rifle that had a pretty crazy amount of thrust (it was made to highlight some sillier aspects of Standard Rules aerospace- specifically atmospheric control rolls). I can't remember where I put the record sheet, but it wouldn't belong here anyways.
In general, I'd say the biggest advantage Support Vehicles have is options. You don't get nearly as many pre-installed free features as you would on combat vehicles or aerospace fighters/small craft of the same motive type, but you do get a lot of choices to pick-and-choose, stack modifiers, and even several exclusive motive types.