Easiest way to find out that there's no balancing issue is by playing scarabs yourself. A good player will easily wreck a mediocre scarab player. If you can get into Tiberium league just using scarabs, then let us know again that it's unbalanced.
I usually throw missile squads at stationary scarabs, then control area with a shockwave. Any scarab on the move will lose to missile squads or shockwaves.
Scarabs are ace!
Eh, I have seen rare cases where they were used well. Like e.g. holding the back pad of a map with three missile pads.
Scrabs are good for niche usage, but then the question becomes: Are they seriously better than whichever other unit you could use instead?