You're right. Milotic's high SpD allows her to take on Draco Meteors without getting too crippled. The main problem is that, if Salamence was in the standard metagame, how come we never saw a big increase in Milotic usage if it was a good check to mence?I've been looking around and found a reasonable check (note the word) to Salamence: Milotic. Yes, I know, it's 2HKO'd by the thing, but still, she pretty much forces Salamence to use Outrage, from which then you can exploit. Draco Meteor does ~60% to 252 HP/252 Def/4 SpD Bold Milotic, and Outrage does ~50%. So pretty much, the only way Mence is going to 2HKO you is to use Outrage (Or it dies), so you can go to your Scizor or whatever. Unlike other bulky Waters who gets crippled by DM, Milotic has reliable recovery (Yes, I know Vaporeon can Wish, but when it's at about 30%, Wish loses its reliability).
I'm thinking Specially Defensive Suicune also be a counter to MixMence since Suicune has defenses higher compared to Milotic.
This also means that common sweepers like Infernape could just spam Flare Blitz/Close Combat in order to kill something, or Dragonite using Draco Meteor like you mentioned.Because Salamence is the next target and takes the least amount of skill to use. You can get a kill with Salamence by getting him in and pressing Draco Meteor, which leads a lot of people to believe hes Uber. However, you can do the same thing with other pokemon, like Dragonite using Draco Meteor...
HP Ice and Focus Blast gengar is also quite threatening. But Gengar plays a completely different sweeper compared to Mence, so the two cannot be compared, even if Gengar is way more unpredictable compared to Salamence. Heck, Lucario is a better late-game sweeper compared to Mence but he doesn't function as well as a wall-breaker of sorts (which mence does just fine). Still doesn't stop Lucario from wrecking havoc on the opponent's team.Gengar is nothing to scoff at either. Those immunities he has take him a long way. In addition, the move Substitute just opens up holes on the opposing team. A moveset of Substitute, Shadow Ball, HP Fire, and Focus Punch is very dangerous. This is more responsible for destroying my core than Salamence could ever dream of achieving.
edit: Mence needs quite a ton of support to work properly, much like Gengar and Lucario. The difference is that Mence is generally better as a wall-breaker/DD while Gengar and Lucario work better as late-game sweeper/cleaners. It doesn't make sense to say that Mence is a better pokemon compared to Gengar/Lucario since they both play different roles.