Now as far as I can see this is by far the greatest discipline of the Old Ones because means Slanny boy cannot be harmed by anything, and it specifically says anything, that isn't magical. I mean, how cool is that? Only their heroes would be able to kill you... so surely all you have to do is pick them off and you've won the game? My concern is that I hardly ever see this discipline in peoples' army lists... Have I taken this single, unavoidably straight sentence the wrong way? Can anyone help?
Beeing a large fat toad, what you fear the most is a cannonball to the face. 90% of the cannons aren't magical, except those carried by the Dwarfs. You also need to look out for magical tricky stuff that might harm you, like spam of the Nurgle's 1st spell and such things. With that in mind, the 2+ save of the plaque of protection VS shooting starts to look more solid AND cheaper. The enemy characters with magical weapons shouldn't be such a threat to your toad if he is sitting confortable on his Charles Xavier chair in the second rank of your X-men unit. Basically, people prefer a more general, stable and cheaper protection than a more expensive one with more flaws.
Lore of Death can still cherry pick the Slann out of the second rank of TG. And a Dwarf Rune Cannon (S10) with a 5pt Rune of Burning counts as magical and can spell bad news for Slann and wreaks Stegs.
sorry, 90% of cannons are not magical? the way i see it, the only cannon using armies are empire, dwarves, and skaven (though the skaven one is a bit different), and 2 out of those 3 are pretty much going to be magic (ok, technically dwarf cannons aren't magic, but everyone takes at least 1 rune just in case) anyway, i think we end at the same conclusion, if you're going down that route, 2+ ward is better, and cheaper, if you're going down the TG route, then you don't really need either. so yeah, either solo slann with 2+ ward, or slann+TG. that's why there's so many lists out there without ethereal slann. oh, also, regarding disciplines in general, i think focus of mystery, focused rumination and becalming cogitation are significantly more powerful than the others, so ethereal would be my 4th choice at best anyway.
You also need to remember that if he finds himself in combat, and subsequently flees, then he can still be ran down and destroyed like normal. Then there is spells, magic artillary etc. Actually, what mundane stuff is normally a threat to Slann at all? Maybe the odd Empire cannon, or various army catapults. As others have said, there are still things that can destroy him and much cheaper ways to protect him.