I agree that mammoth are not unebeatable but they still can kill any Nod unit quite easily. I tried it and I was able to move up until I could see the Nod base. You put a few Mammoth tanks together and you get a pretty devastating squad.Plus, the 600/600 energy that they have means that they don't need constant repairs like a sealth would need.
I played stealth tank too. The problem with stealth tank is they only have 200/200 energy. So you spend half of your time trying to blow up mammoths and the other half repairing... And then you got some stupid morons (from your team) that jump in your tank, while you are repairing it, and they go get it destroyed... but that is another topic I guess...
Bottom line is... GDI has more power but they cost more. By putting some extra credits at the start you give them a little edge. It might be small...but it is still an edge...
See you on the field...
Nik