Imperium fights for 10,000 years with shitty unbelievably inefficient tech. Chainsaws are all very GRIMDARK and cool, but what they ain't is weapons of total war. Their enemies would be who number in the hundreds of thousands at most and only seem to launch fairly sporadic raids (if they were doing it that often, don't you think they might be kind of, I dunno, DEAD after ten thousand years of straight combat?) Core and Arm fight for 4,000 with insanely advanced exploiting/processing/construction tech and fuck the entire galaxy. TA has the resources of an entire world being turned into robots which then junk each other, all in a matter of days. That is much, much more intense and devastating war than anything 40k has, including Nids. tl;dr Either Arm or Core could easily fuck the Imperium or any other 40k faction in a couple of months, tops.
And yeah, war ended with "toast the entire universe except one Core Commander", hell of a contingency plan.