[@Flamelord]
The Tyranid can do the recycle thing as long as they're at their home ground. When attacking a different location, they have no easy way of bringing the corpses back to the factories. And Tyranids can be defeated, but only by destroying their main factory, a tactic the Imperium is well acquainted with. But yeah, all the factions are going to need to work at least a little bit together to beat the Tyranid.