the craft-worlds are a race almost as ancient as the necrons who are out to avenge their creators, the old ones, by killing the necrons to extinction.
Eldar are tricky to pilot, but can still do some very scary things. Craftworlds mechanized lists pair high value shooting with some of the most absurd mobility you can get, with a side of psychic shenanigans. They're hard to master, but not that hard to get a feel for and have fun with.
craft worlds are not 'good,' as they see themselves as being above everything else, & they have no qualms about allowing millions of others to die so that a few hundred of their own species may survive.