Just like how snipers can only attack infantry, and how Melee units can’t attack air. They are meant to be used as building killers, so why are they scarab and condor killers too? They literally kill units better than buildings at this point. Thats not what they are for.
Locusts are supposed to be high damage output and very thin skinned. I agree that they should only be viable against buildings but this is not the way to do it. In all honesty they just need to revert them back to the Halo Wars 1 style, where they can only shoot while they stand still. That way they would still be effective for what they do but they wouldn’t be able to pick off an army from a distance. The idea behind locusts is that they use an incredible amount of power for such a small vehicle and the massive amount of power they use to attack is supposed to hinder them from being defensive in any way, including being able to move while they shoot.