I've played umbrae and gargantuan so far. I feel like as stated before garg is quite movement impaired compared to umbrae, one could argue having good offensive balances it out. Buut so far I've seen is that if you don't somehow suck fifty people in the first 5 minutes and get full power, sec will get weapons and armor that make the Gargantuans offensive abilities pretty pointles. Where umbrae can just teleport away, gargantuan has to try to push through seccies and/or ERT.
Basically being mobile in endgame feels like much better choice than gargs unga skills.
Like when I was umbrae, I could just hide in walls and closets. But as garg I just got fucked over with implanted, riot armored sec armed to the teeth with thermals and immolators. Dunno if any of this rant really helps, but just putting it out there.