This is true, but as a huge Spawn fan, I will say that Capullo eventually established himself as THE artist of the title. His level of ability took off like a rocketship during his run.
You have to remember that McFarlane, with the exception of inking and penciling a few covers here and there, pretty much gave up after issue 15. Then it was Capullo's ship.
I don't see how anyone could knock his work. Anyone who says that Jim Lee or David Finch or Marc Silverstri blow him away...I just don't see that at all. They are all excellent artists with a ton of talent, skill, and ability.