For some reason they count as heavy enough to drive over small walls/debris at cost of their hp.
But unlike really heavy units like bulls/tigers, rocket launch vehicles have puny 1000hp and die to like 3 wall segments crushed. And they are like one of a kind, you usually don't need more than single one, or maybe a few.
They already need enough microcontrol to position and choose targets, to have to manually path their way so that they don't crush themselves without even being hit by enemy.
I think, the best solution is to just strip them of the ability to crush things, since that doesn't help them in any way, but singnificantly hinders their play.