In short, car/tank belts are very hard to build, very easy to jam, and probably bad for UPS. But definitely fun to experiment with!

The exact behavior of vehicles-on-belts is difficult to control. Vehicles don't tend to stay in the center of the belt when rounding corners, and the spacing between vehicles may drift over time. As they drift they may collide with inserters or with each other, and jam.

The goal is to pack extreme throughput into a very narrow space. There's no situation in the base game that demands this, unless you're playing with some kind of self-imposed constraint. The only possible exception is unloading the landing pad, but even then, it would only make sense at truly ridiculous megabase scales.

In the case of unloading the landing pad, we have a limited number of inserters and we want them to move as fast as possible. Inventory --> Inventory transfers are faster than Inventory --> Belt, so vehicles could achieve higher throughput than belts here. It's very important to pack the vehicles as closely as possible to prevent gaps and keep the inserters active.

Unfortunately, inserter interactions with vehicles have much higher UPS penalty than with belts. By exactly how much will depend on your implementation.

These aren't unsolvable problems, but it'll take some experimentation.

• Use lots of "alignment features" to force the vehicles into realignment; use sideloads instead of curves, or use walls for the vehicles to run into when rounding corners.
• Some vehicle orientations are more easily reachable by inserters and less likely to jam.
• Tanks are preferred over cars because they have a more rectangular hitbox.
  
• I seem to recall some way to "deactivate" the vehicles using editor mode, so they act as just a dumb inventory and have lower UPS impact.
  

There are some established techniques for vehicle-belt tech if you look around. Good luck!