During the first few months after Collectors came out, I was convinced that they appeared with greater frequency under "unknown" than the other three factions.
Completing Collector Mastery ahead of the other three faction challenges reinforced this. Nearly completing Collector Mastery twice before I finished Cerberus Mastery the first time further reinforced it.
Even now, I am ahead in Collector Mastery, compared to the other three.
But my perception is wrong, because:
- Everyone will face 1 faction more than the others. It would be a massive statistical anomaly, if this wasn't the case.
- Collectors have fewer unit types, and all of its unit types are represented regularly throughout Gold games (An example for contrast would be Cerberus, where Dragoons replaced Guardians on many waves, diluting unit variety.) This is why I consistently complete Collector Mastery ahead of the others.
In my experience, the faction randomness is pretty evenly distributed, even within a single play session. I still notice Collectors more, but that's because every time I see them, I react with, "Dammit, not these guys, again!"