I played this with my friend and yesterday, and we had some thoughts.
I'll start with the biggest thing, we had fun. It's a great ruleset, the armour saves don't slow it down much at all facing a "peer opponent" adds a whole other dynamic to the game.
Now, onto the nitpicking...
Firstly, we found it a slapfight. We were both using Cataphractii and found no one died. We had one melee fight that lasted like 4 or 5 turns.
IMO AP could perhaps be increased a little. Currently powerfists are AP2 - 3 or even 4 (with equivalent increases across the board) would help things feel a little more decisive.
Also, Tartaros being 7+ - barely better than standard power armour feels very strange. 6+ like Indomitus would be better
imo.
As well or instead, some mechanic reducing armour against attacks in the side or rear (say -1 side, -2 rear) will reward flanking a little more.
Also, some bonus for having multiple people in a combat would be nice.
We also found your charge to
LoS (making it corner to corner) rather strange, you had characters tucked into side corridors firing away as if they had clear
LoS. The base game uses centre to centre which I think is much more logical.
In fact, I'm not sure this is intentional. You give a photo example, with the caption stating the Ultramarine cannot see the Word Bearer. Except he can per the corner-corner rules. So clearly something is incorrect there.
A couple of these suggestions might have reaching effects that would lend itself to being stricter on diagonal actions. Perhaps prevent movement and melee through diagonals?