In strategy mode. A little more testing, this howitzer does massive damage to 3 Warlus in a base roughly 600 meters away. It also works on the turret, but not the garage. The shell cannon on Walrus also does no damage to buildings without direct LOS. and yes like MadDogX said, the shell cannon on carrier is working fine against both ground units and buildings. Probably only those can be targeted could be destroyed by long range howitzer?
I used Manta to spot and adjust fire while a Warlus Howitzer sets up 640 meters away from a scrambler. 6 direct hit on the scrambler antenna dome does no damage. The splash animation is working though. When I drive the Warlus up close I can one shot the scrambler with the Howtizer. The same thing happened when I targeted a garage. Not sure if it's the same against armor/troopers. Didn't test it yet. Sure this big gun works fine on point blank range but a howitzer is supposed to be used somewhere like 15 Kms away from target area right?