The Orks are somewhere between the Imperium and the Tyranids in terms of ease of communication and unification (with the Imperium being the most disjointed and the Tyranids being the most unified).
I'd put the Orks way, way lower than the Imperium. The Imperium actually has the tech to communicate across it's entire bulk (astropaths) and is technically a unified body under one leader (however loose that technicality is); the Orks don't and aren't. The Orks lack the ability to come together racially even if they possessed the desire, which they don't. Gazghkull managed to bring together a large number of Orks on a single planet, and the fact that that's impressive just goes to show how disorganized the Orks are. They cannot and will not unify and they don't want to.
"Look into my eyes, and see your death."
"Let them hate, so long as they fear."