→Mechanicum Understanding of Science
You may think this means the lego-tech of the Imperium (and every other faction) should be exempt from this since it's so simple and basic in design if not actual power. You would be wrong. Look at it this way, one man loads an Earthshaker with a shell nearly as big as he is and it hits like a Nazi railgun (on a railroad, that gun) but over only fifteen kilometers. The mass, force, and energy ''do not add up.'' If it's so large but can be used to penetrate armor with diamantine AP shells (it can and does), then a shell that large should be too heavy for the man to lift at all. If it's light enough for him despite its bulk, it should go waaaaay further than a mere fifteen kilometers. The science of a mere Earthshaker shell is ''impossible'' by our modern understanding of the universe. So, just imagine what kind of stupidly advanced bullshit actually goes into making the damn thing and just what is ''really'' going on with it at a quantum or perhaps ''spiritual'' level. Do you really want to screw around with that?
Take the Lasgun for example. There are almost infinite patterns of lasguns, many developed ''after'' the great crusade as the lasgun wasn't all that common back then. The AdMech understand materials and mechanics well enough to create different stocks and triggers. They understand optics to a decent degree as they can focus the las beam with different barrel lengths. What they don't understand is the power pack, because the power pack is a scary super advanced piece of technology that will not only hold enough energy for a hundred shots powerful enough to kill an armored man, but it can be easily recharged thousands upon thousands of times. And they don't have the slightest clue as to how it works.
There's a quite good reason to that. The ultra advanced science used in the Dark Age of Technology was developed with the aid of AIs and super advanced computers. It's entirely possible that even the scientists of that time didn't fully understand their science and a lot of r&d was done automatically by artificial intelligence's far superior to that of any human's, [https://www.youtube.com/watch?v=R9OHn5ZF4Uo&t with programing so complex no human actually knew how the damn thing was thinking]. Now you can't do this anymore because you know that AIs will try to kill you. In the "Mechanicum" novel the Dragon Caretaker says that the Emperor engineered the creation of the Mechanicum. Why would the ''atheist'' Emperor create a machine cult if not because it was the only way to retain technology that humanity would have no possibility to comprehend anymore once the AIs were wiped out.