[@Flamelord]
Border guard is rows of automated turrets. Space Marines just man the entry points. Your chances of corrupting the automated turrets are significantly worse than your chances of corrupting the space marines. And those chances aren't great. Automated turrets run on a Servitor brain inserted into a turret, and will see pretty much anything non-space-marine as an enemy. 

But yeah, infiltrating the Imperium is a bitch. This faction is based on the 40K Imperium, which was pretty much designed to prevent enemy infiltration. Their only weakness on this front was the sheer size of it, spanning thousands of solar systems. But here, their full forces are fairly concentrated, with about half of them on the ground, and about half of them monitoring from above.