RLS plugs into the compiler so can fetch more info than YCM, which mostly uses racer (which sort of emulates the compiler but isn't perfect).

Moving forward I'd say RLS will end up being better. As much as I like YCM/racer, it will be very hard to make the type info complete without reimplementing a full typechecker.

That uses "racer" for rust I think. It looks like Rust Language Server also uses racer + some other stuff.