rsegal said " Imperial, Chaos, and Dark Eldar technology is inferior to Tau." That is completely untrue. For a lesser example, the Tau personal shields are projected by shield drones and are rather ineffective compared to their Imperial counterpart. Said counterpart can be made as small as a cuff button. For the Dark Eldar, well, I sure don't see any Tau running around with Darklight Blasters, do you? People often make this mistake about the Tau. They think that because the Tau look more advanced than everyone else, that they are more advanced. They are anything but. Even their railguns are, fluff-wise, only about as powerful as a Leman Russ's battlecannon. No, seriously, fluff says that the Hammerhead and the Leman Russ are equals. The fact that they are still equals shows that the Tau cannot reverse-engineer it. Otherwise they would have applied whatever makes it so good to their own tech and thus their Hammerhead would become superior to the Leman Russ. It's not, so they can't. If it were that easy then the Imperium would have been destroyed long ago by the tens of thousands of aliens species it is at war with at all times.
By the way, the Leman Russ is just a re-purposed tractor and it is the equal of the Tau's most powerful tank. That, if nothing else, should drive home how much more advanced the Imperium is.
Ork tech can't really be understood by anyone. Even the Ork who made it. A bit of lore has an Ork shoota cut open and the examiners are shocked to find it is hollow. It only looks like a gun, but the Ork thinks it's a gun, so it works like one. Chaos tech likewise can't be reverse-engineered because it mostly runs on lulphysics and daemons. Necron tech is simply too advanced for anyone but the highest ranking Magi of the Mechanicus to understand. The Eldar use tech given to them from the Old Ones, so they couldn't reverse-engineer a bow-and-arrow. Speaking of which, he Tau would fail at reverse-engineering Craftworld Eldar tech because what isn't run by a psychic presence is too advanced for them to even begin to comprehend.
This is all assuming the piece of tech doesn't happen to be really, really old. Old enough to have a machine spirit that literally twists reality to shoot you in the face with a laser while your holding it in the opposite direction. No, seriously.
To be fair, though, what constitutes "Imperial technology" ranges anywhere from hitting two rocks together and newly discovering fire to flinging black-holes through time and personal teleporters that can be conveniently hidden on your person.