It would not be wholly competitive, but you can have some fun with it: bringing Kharn in a Land Raider and a Lord on a Juggernaut among Spawn, or something. Or bring Warlord Huron to infiltrate a couple big blocks of melee marines. So... you can have some casual fun with it, but competitively? Not going to stand up to Knights, Eldar, a Flying Circus...
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!