Ah, I've seen some pretty epic comebacks. It usually comes from reclaim. If your opponent is being crazy agressive, just focus on holding back his attacks with some defenses and units. If he keeps throwing his units into the meatgrinder it will mean tons of scrap metal for you, if you can survive and get the reclaim that is. If he manages to get the reclaim instead, you will be a goner. The key is to get the reclaim, and to not invest too heavily in defenses. Defenses are really powerful, but you won't be able to push forward again if you invest too heavily.
Another good way for a comeback is a strategic tech-switch that your opponent isn't ready for. Like air for example, if you can do some good economic damage with gunships or bombers (to prevent his building units you only need to stall him in one of energy, metal, or buildpower; if he has lots of mexes everywhere targeting energy and nanos can be good). Or else whatever units counter his army. When the battle is at your front step, your unit-build reactions will be much faster (because of the smaller factory to front distance), so you should be able to have the edge on the rock-paper-sissors type relationships.
Lastly off the top of my head, overdrive is pretty powerful, and if you have less territory but a lot more energy (super fusions!), you can actually be getting more resources, so it would seem rather like a comeback from losing. This method is extremely risky though, because your super fusions will be a delicious target from your enemy's perspective, and the safer you want your fusions to be, the more you will need to invest in defenses. It's generally no where as good as fusion and moho mm spam in BA.
In general though, the larger the teams, the harder the comeback is to make. Your general efforts will probably be dampened by the overall fail of the team. Because of communism mode, a good player or two can't hold up the team so easily. That and given how aggressive-expansion centric the game is, there really isn't so much the possiblity of having a eco player in the back (compared to the aformentioned moho mm) who will suddenly appear with massive army-pumping capabilities. So unless there is some unusually good team collaboration, you probably won't see it in an 8v8. Such comebacks are more commonly seen in 1v1-3v3 and FFA.
