Exterminatus, anyone? Honestly, the Space Marines are so xenophobic that if the Rebels or Empire didn't join them, well... The Imperium would be backing them up, and the lasguns the IG have are pretty much the same as the blasters SW characters have. Eldar would be able to demolish any and all SW ships, Craftworld vs Deathstar, the Craftworld will have stronger ships inside it, honestly, the Eldar have inter-dimensional STORAGE SPACE, the Empire would die. Then let's not even get into if Chaos, 'nids, or Orks wanted to fight a SW faction. Star Wars loses, hands down. Baneblades and Titans and hideous monstrosities that live only to feed off of the life forms of the galaxy, spreading throughout everything until all will be destroyed, oh my.