Personally, I've always been of the opinion that the endless armies of augmented super-geniuses that is the Mechanicus do in fact understand the vast majority of their tech. The best and the brightest can create such wonders as flawless plasma weapons, cranial implants that don't lobotomize you, and bionics that are virtually invisible. But archeotech is where I draw the line.
Personally, I think that making the setting literally hopeless kills the entire point. How can there be despair and fear if there is no hope and you might as well lie down and die? I don't believe the [insert whatever faction you **** over here] should be undefeatable, and I don't believe the technological stagnation of mankind should be irreversible. If they understood those technologies once, they can theoretically do so again.
That does not mean a single tinkerer in some backwater sump hole can just take out his screwdriver, take apart one of the most advanced pieces of technology in the history of mankind on his hours off, and magically understand the underlying scientific principles of its construction. For a real-world analogue, get your hands on an entire robotics factory, try to take the whole thing apart yourself and figure out how everything works, then manually build an entire new factory. And when I say manually, I mean constructing everything from raw materials, since I'm pretty sure the Imperium doesn't have most of the parts for a teleportarium lying around.
I actually did make rules for reverse-engineering archeotech stuff once. A couple of percent's chance of success (at the very best), and an overwhelming risk of destroying the item. I figured you may well be able to reverse-engineer a super-tractor you find several thousand of, but there's a very much finite amount of teleportariums to go around.
Although mechanically speaking, you can of course buy tons of them if you have the PF.