Wasn't a lone, unarmed 8472 able to give an entire Hirogen hunting party serious trouble?
8000 of these guys would wipe out the police and national guard as well as the weaker members of the Avengers (Cap, BW, Hawkeye) without much trouble, that's assuming they don't resort to using orbital bombardment. It's basically Hulk, Thor, and IM vs 4 ships that are each more powerful than pretty much any single Federation spaceship. Hulk is useless in that fight, and neither Thor nor IM have shown the kind of power that would make me think they could so much as damage one of the ships. If you take away the ships and just make it a land invasion then the Avengers *MIGHT* stand a slim chance of winning.