From what I've understood in my entire time playing here, main problem with trying to unite the "four" houses (five, if you consider "Hispania", six if you consider Outcasts and Corsairs as seperate superpowers, seven if you consider Gallia) is politics, in a nutshell.
The biggest trouble I can see are having the Outcasts and the Corsairs joining. I personally think it'll be a moot point to have them together in such an alliance.
The Outcasts' backbone export is Cardamine. If you didn't know yet, it's a drug that alters the body to the DNA-level, meaning it has a near-100% addiction rate (did I also mention there isn't any known cure yet?). The Outcasts are defined by their dependency to this drug, only differentiated from "addicts" in that they're the only producers of it in their homesystems. They earn much of their money by spreading Cardamine to the other houses. Trying to ally with them will create a LOT of ethical and moral complications, aside from the obvious threat of accidentally have them spread their "poison" faster. Oh, and apparently they've got a great deal of reverence towards the Nomads, and the Nomads too look at them with interest (AFAIK).
Picture modern-day North Korea set in a barren Mexico, have the country be irradiated like Chernobyl, an ecosystem consisting mostly of carnivorous plants and animals, and no way to get in or out of the country unless you used cars through natural landbridges that were highly unstable (which, due to this Mexico's lack of mineral resources, can't make their own cars), but at least the place was littered with trillions of (mostly inert) alien artifacts, then put the scene IN SPACE, and you get the Corsairs, the meanest, toughest, roughest nationalistic guns who are very resistant to radiation, and are extremely outsider-phobic. Loyal to their empire, they will NOT hesitate to gut you down should they think you no longer benefit them in any way, and they've got the muscle to prove it. There are exceptions (*coughmycharactercough*), and they "can" form "non-aggression pacts", but such are very tenuous at best.
And then you have the fact the Corsairs think themselves having been "abandoned" by their Outcast brethren to said Hell-Mexico, and you've got a centuries-long war between the two sides. Also, the Corsairs are the greatest "activists" against Cardamine use.
As for the other houses and factions, well... there are other people who could better explain it for me. If there are any Corsair/Outcast players here who can better explain such than my little summary, or if you'd like to correct me, then I'd be happy to be corrected!