Copying over my answer from the thread:
Well fundamentally that's sort of the rub, because there's multiple readings of the source materials, all of which are ultimately equally valid. In many depictions, absolutely WH40K armies are bad at performing expeditionary warfare on a planetary scale. In a smaller number of other depictions they're implied not to be, and some are unclear or arguable. There's simply no consistency, and it's a setting written by people who on the whole are not even armchair military experts.
If you want to interpret Warhammer 40,000 as the Imperium and friends just fundamentally being very bad at war, there is definitely ample evidence to support this reading. If you want, you can find a lot of stuff which seems incompetent to a degree where even the Iraqi army would look like gods next to the best formations of the Imperial Guard. Equally, if you want to say, "this is a civilisation which has been fighting wars for almost as long as human history since the invention of agriculture, and where martial exploits are the single most important achievement for their elite class, their way of fighting war should at least make sense within their social context", then there's sources you can emphasise and readings you can take which do that. Personally I find this fun because it makes the Imperium feel more real and idiosyncratic and fucked up, a sort of mirror to the darkest times in our own history, but... it's not more valid because of that.
To be clear, I think even in a fairly generous view of the Imperium, it would be hard to argue without really squinting at the texts that the average Imperial Guard deployment is much better at war than, as an incredibly rough comparison, Russian forces in Ukraine today. Huge fuckups at the outset of campaigns due to a lack of clear orders and poor planning, different service branches at loggerheads, unsupported infantry assaults in conditions reminiscent of the Western Front, an air force which struggles to do more than basic CAS and gravity bombing... but somewhat ameliorated by a massive advantage in supporting fires which are used as the solution to most problems, and an ability to replace losses. There are probably specific regiments or campaigns which rise somewhat above that standard, and ones that fall far below. Many PDF forces have combat showings which do seem very reminiscent of the Iraqi army circa '91.
But ultimately it's what reading you want to take out of this massively internally contradictory and sprawling set of source materials. No one actually cares about how operationally incompetent 40K armies are, outside of discussions online like this, and it's clearly not a priority for GW because why would they care? Their demographic is a million miles away from people who play Command: Modern Operations for fun, or even relatively soft wargames.
There is no reading of the setting of
Warhammer 40,000 which is so ironclad that it is not going to be contradicted by a single line from Codex somewhere, or a paragraph from a Black Library novel, or a whole page from one of the rulebooks. The Imperium are bad at war, except when they (sort of) aren't, the Tau are good at war, except when they (sort of) aren't, massive orbital insertions and usage of orbital bombardment as standard tactic are not a thing, except when they are clearly described, and so on. So we necessarily have to make choices about what to ignore and what not to ignore, based on what is "reasonable". But this necessarily implies a value judgement on our part.
Even quite common aspects of the fiction can vary a lot in meaning depending on how we interpret them. The Imperial Guard's usage of bayonet charges, for example, could imply a paradigm of warfare somewhere in the 19th century (or worse), or a last-ditch tactic used essentially to avoid being captured alive for cultural reasons, similar to many banzai charges by Japanese forces in WW2. Or even (being very generous) an infantry tactic to infiltrate and encircle foes who will broadly win a straight firefight like the Tau, similar to Chinese tactics in Korea. None of these is
good in terms of what it says about the Guard's overall limitations, but each has meaningfully different implications for how (in)effectively we expect the Guard to fight.
So I think before we ask this question, we have to sort of interrogate what view of the
Warhammer 40,000 setting we want to take first, and why we find that reading valuable or interesting first. I realise this is kind of a non-answer to your question, and I apologise.
But I think it's sort of necessary, because we have the situation you see in a lot of Versus debates, where the argument basically flips between two levels simultaneously; there's the diegetic layer focussing on how the fictional property interacts with the opposing side, and the non-diegetic layer where people usually have diametrically opposite reads of the fiction, usually solely based on how this makes them good at fighting or not:
"I think that the Care Bears could defeat NATO forces in the Fulda Gap if they were swapped in for WARPAC forces."
"No, I think they'd totally get owned."
"But a Care Bear Stare is described as having a yield of somewhere between 5-100 kT, and as seen in Care Bears II: Revengeance, Cheer Bear is able to deliver hugs to all the children of Grumpy Gulch in under 2 seconds despite being behind a ridge, implying an ability to non-visually identify and track hundreds of targets, whilst in Care Bears IV: The Reckoning we see Funshine Bear, one of the weakest of the bears, managing to hit a target in low orbit across the horizon-"
"That stuff is all stupid and doesn't make sense. In an of the episodes of the cartoon, the Care Bears get beaten by a small cat, and in the 1990s text adventure game, the Care Bear Stare can't get through a locked wooden door."
"Sour Puss had been empowered directly by the dark sorcerer Mordred, and that door was made of-"
...and so on.
Once you've accepted that a value judgement is involved somewhere, I think you have to interrogate why a given internal model of the fiction is interesting in the first place. Or to centre it directly in terms of what is relevant or not for a discussion of practical fighting capabilities; I think the only way this conversation does not get fundamentally repetitive by the second or third reply is to talk much more explicitly about the exact models we have of the fiction, and why we have them, etc..
