The COP may not be fully human, but they do capture human planets, with human civilians. That means free cannon fodder.
About this creation
The conscript infantry of the second line COP armies is specially brainwashed and utilized in a surprising way: they are completely fearless, and every facet of their equipment is purpose designed to scream 'I'M HUMAN!' in order to temporarily prevent UNE soldiers from firing apon the fanatical conscripts.
Experiments have proven that the COP brainwashing techniques are incredibly effective and almost totally irreversible; even in the few events when conscripts can be turned, they are equipped with centuries old equipment and stolen UNE weaponry, certainly not state-of-the-art. They are told nothing of importance: the most complex order a COPCON, as they are called, will ever hear is liable to be: 'Go there and do ____'.
They are totally obedient and utterly expendable: even moreso than COP regulars.
Revolt among their ranks is unheard of.
They are poorly trained: at most, they are taught how to fire their weapon and operate their suits; at worst, they are given weapons, told to point them at the enemy and sent into battle.
The power armoured infantry of the Zenexabas Thirty-Third Expeditionary, on the other hand, are trained well, to say the least: each and every one of them has had at least five years combat experience, all are trained every detail, every feature of their equipment. Each and every one can dissassemble, clean, adjust and reassemble his chosen weapon in the dark, each can set his armour to suit his personal style and all have access to every available secondary feature and modification supported by their weapons and equipment. Not a one of them is raw and untried; they are well equipped and supplied, and have immediate access to all weapons of the Empire, be they experimental prototype-undergoing-initial-combat-trial weapons or longstanding standbys of the Empire, like the AR-40,000 assault rifle, the NAYI-5 handgun or the HEIF-2 grenade.