Post: This question grows out of the discussion Faisan and Furby got into above about the nature of the aliens and rebels. How tough are those aliens, anyhow? Maybe it's late. Maybe the heat is getting to me, but I'm confused about how tough and powerful those aliens are. We know that they 

1) have been lurking around for a zillion years, but haven't done more than the random act of violence; 

2) have been in regular contact with humans for fifty years about their colonization plans (all talk no action? Don't we all know people like this?); 

3) need humans as slaves - either to serve as clones or hybrids or hybrid/clones or to incubate more grays; 

4) still, depended on humans to create the perfect hybrid (yet extracted ransom in the form of family members before giving them the materials to work with), even though they seem to have superior technology; 

5) have apparently lost the upper hand to the rebels (according to Krycek in One Son) though we don't have much sense of what has given the rebels the advantage (fire sticks? technologically advanced charcoal lighter? Help me out here.) 

6) have really unpleasant childhoods. 

Seems to me there is some fatal flaw in the aliens that is holding them back and forces them to depend on their human allies. Is it that there are only a few of them seeking to master many (maybe their own planet blew up and these are the remnant)? Or do they suffer from some fundamental physical flaw that weakens and/or dooms them? Is the black oil their death force as well as their life force? What kind of weapons do they have? Do the young cannibalize each other? Are they unable to reproduce except through a captive host? Do they lack a sense of irony? Are they confused by pro sports? Well, the possibilities are endless. But what is  braking what should be relentless invaders?