I don't have NS2 but I watch this guy LP it whenever it shows up on his channel. Here's the best comeback match I've ever seen:
http://www.youtube.com/watch?v=FvmDtVTuIPE&list=UUN-Klifn9C7kINwpIA0uOHw&index=34. The turnaround comes at 58:00.
Galm is Alien, the Aliens pretty much get hammered all game, at the end everyone's talking about quitting, but at the very end the Marines push together too far and Aliens rush into their base and smash their upgrade structures. As soon as Aliens realize, they start fighting back with spirit and demolish the Marines within a couple minutes.
One thing you may not like about Galm as an LPer is his continual use of the minimap and you may get irritated at some of the dumb shit that comes out of his commentary. I think it's hilarious. At one point in an LP he said something to the effect of "ok guys we got some rocks to shit on" out of nowhere.
Anyway, I agree with giving the game a chance since there are such frequent patches. Everything was up in the air for a long time during beta, with abilities going from one alien to another, damage and cost for stuff varying, etc.
I also agree that it seems like a good comm makes or breaks the match. It's easy for a comm who isn't totally on top of it to be two upgrades behind the other comm and you're out on the ground fighting with inadequate stuff. Maybe your marine comm spends rez early on medpacks and ammo and he should be focusing on structures and research, or he doesn't recycle structures soon enough. Maybe your comm drops structures before the zone is safe.
Luck plays a major role too, whether you manage to slip through to a base and do some damage while everybody's busy elsewhere. Did you stay on target on that skulk (which is really the difference between killing it in a clip or nicking it just before it eats you). Did that skulk happen to pop out of the vent you were looking at and you blast it?
And footsoldier skill matters a lot too, whether you can keep your structures up and destroy your enemy's, whether you can keep more rez flowing into your team than into the other one. Maybe the marines keep hitting your cyst chains with flamethrowers. Maybe their exos have excellent welder support. Maybe the aliens keep the marine structures from being healed with bile bomb. Maybe the people in the base get confused and run off in the wrong direction for that 3 seconds you needed to get enough chomps on a structure to drop it. Do you stay out in action long enough to be useful but flee and heal up to preserve the personal rez you spent?
The maps are very interconnected but yet very broken up, especially from the marine perspective because they move through vents so slowly. There are so many opportunities to play mind games with your opponent and you have to think about the map how each side experiences it.
It's a complicated game, which is why it's fun to watch and listen to commentary.