The difference between this and Shedinja is that this gimmick is one-time use only. If for whatever reason the Kyurem takes any sort of chip damage and fails to sweep the first time it sets up, there is a good chance it will never find the chance to do so later in the game since the Sash is no longer intact. This is of course beside the fact that any sort of hazard immediately ruins your gimmick, but since hazards also screw Shedinja over I won't be talking about that here since the topic is a comparison between Shed and Sash Kyurem.
In short, Shedinja is way more consistent despite its glaring weaknesses given proper team support. Kyurem could have excellent hazard removal support but even some chip damage from a U-turn pivot would give the Imposter a green light to go in. At best, Focus Sash on this Kyurem is a gimmick set that would work maybe once vs. a good player.
Edit: Nvm my post I just read this:
But that's like me bringing up Shedinja being weak to Sandstorm, or Status like Toxic, Leech Seed, Infestation, etc., something that doesn't threaten K-W as much.
I don't see how a Specs Kyurem is not as badly hurt by Stealth Rock as Focus sash. Yes, it has immediate power, but again, we are not talking about something frail like Deoxy-A which needs Focus Sash to take even 1 hit, the EV max also benefited K-W, and it was fairly bulky to begin with.
Couldn't K-W still sweep without Focus Sash in fact? It's not an easy KO.
Focus Sash is there, and helpful when it works but it isn't always needed, it's like Lum Berry is there for Shedinja but isn't always needed because other items like Protective Pads or Safety Goggles (for weather), are options too since status doesn't always come into play (W-o-W's nerf makes it less common, Gengar/Mega-Gengar reduce Toxic Spikes' effectiveness due to absorption)... In fact, I have successfully used Misty Surge on a Shedinja team to compensate for lack of Lum Berry, and it worked well enough to let it use Protective Pads.
I see the bigger difference being the set-up move not the item... The item assists with the set-up move, but it isn't required. Imposter can be stopped by Focus Sash letting it survive, but there is still the Speed tie, just like there is with Specs K-W if it comes in at less than 100%.
One thing not considered is Imposter coming in on the Quiver Dance... not only does it prevent a Speed tie, it also means you will hit harder than the Imposter user, and threaten it, or hit their switch in harder than before.
If you have Prankster Spore, Haze, Heart Swap, Topsy-Turvy, Unaware Registeel/Gyarados-Mega, etc. then you can also handle Imposter.
Let's not assume worse case scenario with Focus Sash, you play well, you let it come in at 100%, set up on their switch, sweep, they send in imposter, you survive the hit while hitting them, etc.
Let's even take the set up move as the main difference; if you can use a Choice item that gives 1/3 the benefits of Quiver Dance, then QD seems like a decent choice.
Again, K-W + 1 Secret Sword vs Imposter K-W 0 IVs - Defense nature is the aim, as Quiver Dance makes Core Enforcer less effective against an Imposter.
Kyurem-W can also counter Kyogre-Primal better with Quiver Dance allowing it to sponge water hits, and boost Secret Sword to hit Kyogre from the Physical side (even if Kyogre Quiver Dances alongside it), and K-W's typing removes the Weakness to Ice most Dragon-types experience. Quiver Dance offers it things that a Scarf/Specs set does not, namely Bulk, move and item choices.
Oh, and what if it gets more than 1 Quiver Dance up? It has officially reached beyond the Choice Item benefits even more!