So, in my newest restart I decided to just cheat and make a broken army, partially so that I can hunt for bugs at my leisure. I created a cheat army with all stats maxed and the traits Kings of Space (because space combat, duh), Slow and Purposeful (survivability), Ambush (to off-set the damage decrease of SaP and also make them better at raiding) and Crafters (you'll see soon).
So, new game. I fired up cheat engine and spent the first hour or so outfitting my army with Tartaros and Artificier armors (I got those through the Crafter trait and then used CE to create 3000 each).
-Terminators, HQ and Chaplains/Apothecaries: Tartaros + Hammer + Assault Cannon + Rosarius
-Tactical: Artificier + Power Sword + Plasma Gun + Rosarius
-Assault: Artificier + Power Axe + Storm Shield + Jump Pack + Rosarius
-Devastator: Artificier + Rocket Launcher + Plasma Pistol + Rosarius
-Scout: Artificier + Sniper + Power Sword + Rosarius
On top of that I gave each company enough Predators and Whirlwinds to set their size exactly 600 and loaded them into 10 Battle Barges divided into three fleets. The cruisers/escorts of the original fleet became my bombarding fleet.
Once I set up everything, I let this army loose in the sector and managed to steamroll everything. Orks were no problem because of the vehicle-bug (if your first line is Predators/Rhinos/any kind of vehicles, they don't attack), Tyranids could be massacred in raids (their first line always consists of cultist and genestealers that could be alpha-striked on the first round, after which point only armoured limousines are left, which are supposed to be just damage-soakers and can't hurt your marines). The only chaos world I found I immediately bombed back into the stone age, while I didn't even encounter the Tau and Eldar. Oh, and I also cheated myself full of all three kinds of STCs, which by pure chance all gave me armor and survival upgrades.
Because of all this I managed to pacify the sector without suffering a single marine casualty. That however doesn't mean I had an easy time.
Now come the bugs:
-The feasts not only don't work, they routinely crashed my game.
-I had to unload and load my companies a few times at the start because sometimes the game refused to acknowledge them, but once I did that I had no problem with units not participating in battles anymore.
-My game had no Inquisition. This might not have been that big a deal, but I also happen to have 15 Daemonic/Chaos artifacts in my Librarium I cannot give to anyone and I also couldn't get Cycloninc Torpedoes and Exterminatuses. [Edit: The first inquisitor appeared around turn 150. Now I have Exterminatus but I still have no Cyclonic torpedoes.]
-I had several CTDs during exploring Ancient Ruins to the point I no longer bother with them.
-A Necron tomb awakened... which happened to be on a Forge World. No biggie, I thought. I happened to be right there and they were still at "Heavy", so I thought I might be able to nip them in the bud. I couldn't because the game crashes when I try to engage them. Currently I am entertaining the thought of letting them massacre the entire population of the world and then bombarding them into oblivion. I am practical like that.
-I double-confirmed the Iron Halo bug. I had an entire company equipped with them, and sure enough, after one battle they were all down to 80%. Worse, if it is combined with Bikes (which give units +25% health in theory) they immediately go to 60%. Right now I am looking into whether Bikes on their own also cause health loss after battle.
[Edit]: New bug: Two of my Battle Barges are stuck on two different planets... except they are not. My fleet is at planet A, but they are registered as still being at planet B, they can only unload their companies at planet B and can bombard planet B, and conversely they cannot actually be used to attack/raid/bombard planet A, or any other planet if we are at that.