Definitely feeling this when I play GDI, I almost dread fighting Nod now as it seems a lot of the lower tiered players have the Rockworm. The problem currently is that as GDI your only options to counter it are GDI Missile Soldiers, GDI Jumpjet Troopers, GDI Zone Troopers, Commander General Soloman's Ion Cannon (150 cost to kill a unit of 120 cost), GDI Orcas (these are a problem in it of themselves but more on that later) and GDI Kodiak. However what I don't like is that while anti vehicle infantry are supposed to counter it, when it emerges it all but kills the squad that is supposed to deal with it, this leaves the GDI Orca and Kodiak as the only hard counters to it.
Another problem I see is that it is invulnerable while traveling which takes out a lot of counterplay to the unit itself as it cannot be intercepted, it is all too easy as Nod players with the Rockworm to just create the unit and send it over to the GDI Harvester line and wreck their economy while boosting yours as it two shots the harvesters from one attack and the surfacing damage it does.
Solutions I could think of the moment are since it is roughly the equivalent of the GDI Titan increase its cost to 130 Tiberium, give it a greater interval between the time it takes for it to burrow, remove its AOE emergence damage or decrease its movement speed to slowest as it is tunneling under the ground this would be very slow logically. A combination of these ideas would go a long way to making the unit less frustrating to fight as a GDI that currently has no access to anti vehicle air in the form of the Orca or Kodiak.