Vehicles are weak as hell in comparison to Monstrous Creatures. Vehicles have (assuming no invul) exactly one thing protecting them from getting damaged, which is their AV (equivalent to toughness). They then have hull points, which are effectively wounds (most have 3), but IN ADDITION, they can be shaken, stunned, weapon destroyed, immobilized, or explode (instant death....).
In comparison, MCs have high toughness, high wounds, armor saves, they get cover by sticking their toe in it, and they can move further while firing two weapons.
If you compare straight across, toughness 8 (for example) is approximately equal to AV12 (requiring a S8 weapon a roll to pen/wound of 4). For an MC, some fraction of those may go away from armor, or from cover (which is comparitively easy to get), AND MAYBE AN INVUL AND MAYBE FNP. Jeeeeeeeeepers. I compared a DC dread to a riptide once, and I cried. A lot. Anyway, I think some sort of fix to this would significantly balance the armies that have OP MCs (GK, Tau, Eldar) against the armies that have really nerfed vehicles (Marines..... Guard? Do guard move? etc.)