Well, regarding the situation between GDI vs Nod, I kind of agree that GDI is in a tight spot... If only they're upgrades were worthwhile.
The Mammoth doesn't need the predator upgrade cause its firepower is already strong. The issue with the Mammoth is that it would easily succumb to both rocket-fists and busters. My suggestion is to buff its upgrade to a whole new level. Instead of it being able to crush walls, it should be able to crush all sorts of barriers and infantry alike.
It will slow down against rocket-fists, but most infantry should be flattened under its threads. Also the Mammoth is so dense that it would dent anti-tank barriers with ease. I mean, can any vehicle survive running straight into anti-tank barriers?
I don't really have issues with Kodiaks cause I never got a chance to use them. Mammoths I used them before on an very old world (never got its upgrade), but I do understand how their upgrade is supposed to function.
The grenades that Riflemen, Pitbull and Orcas have are quite annoying. You can only use them once per unit in each attack. I would suggest either scrapping the upgrade entirely or buffing it to three charges instead of one.
Also as a whole, I noticed that GDI has a lack of RT reduction upgrades (like shields and so on) and Nod has a lack on range (GDI vs Nod, 7 vs 5, venom doesn't count as it would still move forward regardless of the situation).
Some alternate upgrades would be ideal here, but they would increase by 25x if one of the upgrades is chosen. I just want some options on what o research, like it would be somewhat mutually exclusive, if you pick one, the other would take a longer time to research.