Sidenote re: plot armor; what exactly do you think luck in 40k is?
There is a profound difference in 40k between luck, "luck" = warp shenanigery and Wardian plot armour
Loud Whispers...you realize that superior logistics/efficiency is the only reason the Tau have survived this long against the Imperium, right? The Imperium very specifically does not have a safe means of FTL travel. The Tau do. They don't dive deep enough into the Warp to get Daemons. They have to plan farther ahead to have good logistics, due to the slower speed, but in terms of running a galactic empire? You want reliability and not losing ships to the Warp every fifth convoy. They also aren't saddled with self-serving planetary governors, the inefficiencies and bulk of the Administratum, or worries about the Astronomicon, since their travel sticks to realspace enough to navigate without it.
The Tau can travel long distances faster than light, the Imperium can go from one end of the galaxy to the other with 0 distance traveled in realspace, there isn't really any comparison in who has the superior logistics. There is a reason why the entire Tau Empire could fit inside a single Imperial subsystem, whilst the Imperium is made up of the entire extent of most of the milky way with an uncountable quantity of worlds which they nonetheless futilely (yet must to maintain tithes) keep track of, because the adminstratum is grimdark like that. The Imperium has the second most-efficient method of warp travel after webway gates (whose usage is limited to those who have found/know of safe routes, with Imperials actually managing to occasionally find and use Eldar webway gates without getting flooded by demons or dark eldar). There is a reason why the Imperium is measured in control over stellar systems, not continuous control of territory as the Tau are.
This is the Imperial controlled galaxy
During the Third War for Armageddon, all 9 first founding chapters sent several companies (and the Black Templars diverted three crusade fleets) over this distance:
*On review, I missed out the Salamanders and Imperial fists, but there's enough on there to illustrate
That's excluding the other 14 chapters from other foundings sent. Also all three Inquisitorial branches and their three militiant branches got involved, with SOBs, Deathwatch, Grey Knights and Assassins running about. The most distant Imperial Guard faction to send brigades as far as I can tell were the Death Korps of Krieg (Krieg being roughly half the distance to Armageddon as the Ultramarines or Blood Angels were), suggesting the Imperial Guard has around half the response time of the first founding chapters when it comes to redeploying to another warzone, which makes sense given how many more people the guard are organizing (and it makes sense the Death Korps were the most distant still capable of responding in time given their grimdark organization times). Excluding the Armageddon regiments already present:
67 Regiments and 17 battalions of Guardsmen (tank formations varying by regiment), 18 companies of storm troopers, 12 penal legions, 9 batteries of artillery, 3 Regiments of Ogryns and 3 tank legions were sent by guardsmen from other systems. Couple that with the 23 chapters of space marines, the 10 companies of SOBs, the 7 Titan Legions, 4 Ordinatus Machines and 14 Skitarii regiments sent - sending all this war materiel across 6 times the length of the whole Tau Empire without losing lines of communication with their origin points; it's incredibly easy to see whose logistical capabilities are superior. And that's whilst excluding the forces that were already present upon Armagaddon. With the Astronomicon allowing for an
astronomical level of logistics to take place in rapid fashion and the Astrotelepathica allowing an Imperial world to quickly raise alarm from one side of the Galaxy to the other, vast quantities of resources can be redeployed at all the chaos incursions, secessionists, xenos raids, invasions, tomb awakenings e.t.c. there is no contest.
It's telling for example that in the preparation for the Great Crusade, despite having all the manpower, material and spehss mahrines needed to conquer the galaxy; it could not begin until the Terrans completed construction of the Astronomicon. Just as the Imperium could not expand further into the Eastern Fringe because it was beyond even the light of the Astronomicon (and the true limit of their logistical reach), the Tau have reached the limit of theirs - lacking a ready source of telepaths and navigators from their own people to make speedy warp travel and communication possible.
This is why the Tau expand in spheres, because their travel goes through realspace, whilst the Imperium's logistics goes beyond realspace and time into some quantum bureaucratic fuckery that nevertheless, makes it all possible.
The black squiggle is the space crossed and time taken for both forms of warp travel (obviously not in any way quantifiably linear).
The Tau engine slingshots in and out of the warp, never really being in it too long to be dangerous, just enough to ride on warp currents and make it ftl. Yet every time they're back in realspace they're following the laws of physics again and this slows things down immensely. Gothic spaceships on the other hand only have the distance they need to get away from habitated areas and the distance they need to fall short of habitated areas to travel in realspace before they enter/exit the warp. Once in the warp as long as your navigator doesn't die you'll be able to take the currents quickly without interruption and arrive at your exit point having not traveled through realspace at all, which is why a journey that should take Imperials 100,000 years takes a few days (or if warp fuckery gets involved, makes you arrive earlier or later. I wonder if an Imperial has warp jumped out and met their future selves yet). Of course the obvious downsides being to be "safely" immersed in the warp your ship needs a gellar field and a warp drive, meaning you can only travel in bulk. Given that the Imperium only knows how to transport in bulk, not much of an issue :
P
And this is all why the webway gates are pretty much the only thing that could one up this... If it worked. Small (on the cosmic scale), efficient, doesn't require personnel to man it, doesn't need gigantic gellar field or warp drives, can transmit in bulk or small quantities in times shorter - would be pretty damn neat if it wasn't broken to hell and all cryptic / full of mindrape / evil robots / sadist space elves / a literal god trying to eat your souls
I suppose the latter is a moot point if the warp is full of four of those though, so the only real issue is it's exclusive to those who have the maps :
D