
why weren't the aliens using the weanpos to revolt instead of selling them to the Nigerians?
The aliens did not stage a revolt using their superior technology because they were stupid drones who lacked initiative. This was all explicitly stated near the beginning of the movie, and repeatedly demonstrated throughout it (e.g. trading the mech-suit for a hundred cans of cat food).
I could totally buy the lack of any organized resistance, but come on... the aliens had totally bad-ass small arms, and lots of them. And there were multiple scenes showing the prawns as tempermental and violence-prone. You'd think in a population of 2 million+ there'd be a whole lot of alien ass-kicking going on. It would be very dangerous to be one of the Nigerian overlords in that kind of environment. Give a stupid person a regular gun and they're dangerous... give a stupid person a gun straight out of Ratchet and Clank and you're in BIG trouble.
But the plot hole that I hated the most was the "one substance does it all" trope. The same black fluid that powers a starship also (very coincidentally) performs a perfect genetic-level transformation of a human to a prawn? I can suspend disbelief like the next guy, but that really pulled me out of the movie.
"It is better for civilization to be going down the drain than to be coming up it." -- Henry Allen