Man, I've just been playing the GDI campaign for the sake of the cheesy cutscenes and several times I've tried to do the Mammoth missions with other units, I have run into a wall every time, especially with the latter missions (against Scrin), it seems that the best tactic is to hold fort with vehicles supported by the deployed rig while amassing the Mammoths. When you have the critical mass of them (about 9 or 10), u strike and wipe everything out.
On the second last mission against the Nod I have actually tried to use infantry and lost the game, when I tried tanks and APCs I won with ease.
CnC 3 is not half the game Generals was, in gens you actually had 3 distinctly different sides (not carbon copies of each other with cosmetic differences) and infantry was useful at least in base defense (u try taking on a bunker full of rocket soldiers with an Overlord: bunker 1-overlord 0) and Zero Hour was a lot of fun and variety.
I think this game has the "Doom" syndrome: it relied on a base of fans to sell inferior product and like Doom 3 I for one am too bored to finish it.