I'd hardly say that it makes helis invincible... given that raveshaw/syndey can take them out pretty well. Heck, any good tank pilot should be able to get a couple of shots in before they get too close. A tank shell does some nice damage to a heli. Nod has the advantage against helis with the light tank. It's turret can point almost straight up and it's fast enough to avoid missiles from the helis. You just have to watch them for a bit until you can predict their movements. Nod also has those black hands that will tear apart any heli that gets close enough. (Great for base defense against heli rushes.

Targetting with a heli is horrible, with how much they lean when moving forward and backwards. Maybe that's why I catch them hovering still often enough to pound them with tank shells. I usually only pilot transports for dropping off engineers/beacons.