Why are aliens bastards? I can sum that up in just one word.
Crysalids. Fucking crysalids.
That said, they're also stupid bastards. Instead of bringing their full force to bear on earth from the very beginning, they instead sent in their forces piecemeal, starting with only weak, easily captured scouts and only sending in progressively stronger forces when the humans have had time to capture and reverse engineer their tech. By the time they send in the battleships, the humans are bringing them down with plasma-cannon equipped UFOs (well, not really, since they are in fact identified, but you get what I mean) of their own and soldiers armed with flying power armour, plasma weapons and psi powers of their own and supported by a plasma cannon equipped mini hover tank.
If they'd just broken out the battleships, crysalids and ethereals from the very begining, Earth would be powerless to defend against them and would be theirs within a month. Overwhelming force, guys, it's not a difficult concept. We earthlings figured it out almost as soon as war became a thing.
But yeah, fuck crysalids. Those things can eat a thousand dicks.