We still don't know their what their capabilities would be against the Flood's. I don't really know of any other "abominations" that could reach the scope of the Flood infection beyond that of Red Harvest. Also, the Haloverse as a whole seems to be much more versed against such threats, while (unless you know of something that the entire galaxy of Star Wars has had to fight) Star Wars might be caught off guard, since Red Harvest and Death Troopers (from what a very quick glance could tell me) were somewhat isolated and very small in scope.