Larger WAAAGH!s that have Kroozers can easily carry a dozen Stompas or smaller Gargants, to say nothing of Deff Dreads, Killa Kans, Battlewagons, etc. Considering Roks are just hollowed out asteroids, the limit on what it can carry would just be limited by it's size.
Hmmm how would Zerg fare against Ork Artillery? You could probably fit plenty of those on a Rok.
Given how well the Zerg fare against Siege Tanks, I'd say artillery is a problem for them.