Well, for one, understanding your technology doesn't really mean jack shit if you have one AK-47 and the enemy has 500,000,000,000 muskets.
Secondly, drone soldiers are ineffectual, unreliable, and expensive. Those material supplies and construction times? That's what kills it. The Tau are miniscule, even the full concentrated industry of their entire empire is negligible in comparison to their only actual neighbor, the Imperium.
Thirdly, Hunter Killer Missiles. The Imperials have them too, and like the Tau's missiles, they suck.
Fourthly and finally, the Imperium does use xenotech, it just has to go through enormously complex channels to be canonized with the Tech-Priesthood, which takes millennia if it happens at all. The Inquisition and the GKs certainly do, and Needle-guns/digi-weapons are of xeno origin as well. Primarily, the Imperium doesn't do this because it wouldn't be cost effective. Logistics are strained enough without them stopping to retool all the factories.
So in actuality, all of their advantages are... Quite relevant to their situation and taken fully into account, they're the only reason that they still exist at all.