I suppose the Devs don't want systems in place to mitiage too many Marine players, as to not upset Jimmy Marine?

The Green Trinity.
Orks, Biel-Tan & Death Guard just destroying SM fortifications across the land.

Goffs, Black Legion and Ulthwé: The Titans of War.
Evil Sunz, Word Bearers and Saim-Hann: The Marauders.
Deff Skullz, Night Lords and Iyanden: The Dark Terror.
Bad Moonz, Iron Warriors and Beil-Tann: The Iron Storm.
Blood Axes, Alpha Legion and Altansar: The Shroud.

Blood Axe Kommandos and Alpha Legion Headhunters tag-teaming the Space Marines would be the sickest thing ever.

... Are you down? I'll main a Kommando Evil Sun, so I'm up!

I'll be maining an Emperors Children Sorcerer when they come out, so most likely ill be hanging out with the Goffs if anyone.

...Not that we lack an Alpha Legion group, it's just hard to tell exactly where they are.

It's not so much where they are, but who they are.

I mean, I could be an agent of the Hydra. Hypothetically, of course.

Now we have to perform a murder-suicide!

there is many gameplay feature for mitigate faction difference in population: the first is the tyranids, stronger faction have their territories attacked more by npc tyranids. also the campaign objective aren't only "conquest all the map" but different objective for each faction, a bigger faction will have harder objective. etc etcetc