There is a bug where boarding parties from different ships are not counted together, but separately so you need one ship to have enough boarding points to take over the target ship.
Just tested with the alltech cheat and still seems to be broken. 1 dreadnaught 4800 boarding strength vs. 1 freighter 500 crew strength. I still lost. A conundrum.
Definately seems to be broken. I can find no combination of attack to defense that takes the ship. Is it just me or is anyone else having this problem?