On the battlefield, they're a stolid grey wave of humanity. They live with a disturbing lack of wanting to live, and are well known for pessimism, morbidity, and an institutional death wish - if you can even get them to talk, that is.
On the battlefield, they're a loose conglomeration of whatever the Planetary Governors can scrape up. In theory, they defend their planets from external and internal threats, forming the first line of defense for the Imperium of Man. In practice, the only tactical consideration they are in the minds of battlefield commanders is a force to slow down threats with sheer numbers until the Imperial Guard arrives to start the battle.
The Death Korps of Krieg and the Planetary Defense Forces are wildly incompatible, and so are their members. However, when misguided aim lands one human from each in the far north of Equestria, teamwork is the only hope for success.
For Rhia, it's difficult trying to preserve one's life when one's survival partner considers you useless, along with not caring about trying especially hard to survive.
For 57-893, it's annoying when your only company is obsessed with the ridiculous notion of not wanting to 'die for the Emperor'.
A change of outlooks is in order.
And what of the natives?