CG is certainly right about IA being broken, though he's off about it being uniformly overpowered. The problem is that there's a lot of underpowered and terrible chaff concealing a few absolutely gamebreaking problem units (Such as Lucius-Pattern Drop-pods and Earthshaker Guns). If you know about these units and make a concerted effort to include them, you'll crush any codex army that doesn't know exactly what you're fielding and tailor itself entirely to fight it, something many regular armies aren't even capable of doing.