What makes Rotom-W a real challenge to handle is that nothing can really switch into it effectively-- not consistently anyway. As a support Pokemon, or a tank, this is a unique and defining characteristic not shared by Jirachi, Heatran, Mega Venu, or any other defensive Pokemon.
It just has to do with how its attacks line up type-wise.
Volt Switch provides STAB, so it's chip damage opponents can't really laugh off-- but if your Rotom counter gets hit by it as it switches in, they immediately get to switch to a counter. This is totally different from say, Jirachi's U-turn or Celebi's Baton Pass because Volt Switch isn't being used as a predictor, but one of Rotom's best go-to attacks that it will use to check its own targets (especially in this meta, where those targets are mostly flying types), and does significant damage. So to truly be said to be a good Rotom-W switch-in, you almost have to be IMMUNE to Electric, as opposed to just resist it or be able to shrug it off.
Moreover W-o-W's burn is a nuisance to the majority of enemies, especially potential switch-ins, so immunity to it is preferred as well (unless you're going to try and constantly heal it off all battle).
However whether it's electric immune Ground-types or Burn-immune Fire-types, both take a beating from Rotom's other STAB-- Hydro Pump. Even for the odd Ground-type immune to water attacks, they have basically no way of hurting Rotom-W back... Hidden Power Grass Gastrodon, lol
Throw in the truly threatening risk of Trick (which often cannot be scouted easily due to the popularity of Chesto berry), and you're facing a real challenge into switching into this thing-- far more than any other defensive force in the meta. By the way, the threat of Trick is one reason that makes Chesto Berry so good on Rotom-W specifically, and why Chesto Rest is seen on Rotom and not nearly as much on other Pokemon.
Rotom's attacks and typing fit so well together that there basically is no answer-- and the metagame has moved along just "enduring" rotom's assault because there's no way to actually prepare for it well.
Frankly, I wouldn't even mind seeing Rotom-W suspected.
edit: (though obviously, not until after a whole bunch of other things got banned, including the A+ ranked Talonflame, and Rotom-W's ban would probably make Lando-I just broken)