Sorry to spoil it guys, but there is definitly a unit cap.

Look at the symbol in the middle, it is a picture of a man, obviously a pop cap. The rhino looks decent in stats, but fairly expensive compared to a firepower supperior vehichle.

As you see here the scorpion is a far greater unit in almost every single catagory, but is much cheaper. The conclusion? There is no doubt that the rhino is a long range artillary unit, or else not one person would waste their resources on it instead of a scorpion.
Stay tuned for a hardcore analysis.