MegaDance's issues aren't with priority as much as they are with mediocre bulk. 50/110/110 really actually isn't that great, especially when most can't afford defensive investment. I don't find her overwhelming for offensive teams because she has lots of solid offensive checks, and most threats that she can't OHKO can hit her back hard. Stall can usually deal with her as well, although the CM set can usually do some excellent damage to a stall core in the process - Blissey will generally get in two Seismic Tosses before going down, and with no recovery, that means just about anything will KO.
Just looking at the viability rankings, S and A ranks...
- CROBAT: OK, yeah, Diancie pretty much craps on standard Crobat sets.
- JIRACHI: Scarfrachi outspeeds, Iron Head always OHKOs, and even 0/0 Jirachi takes at most 79% from Earth Power.
- SUICUNE: 0 investment Scald 2HKOs with plenty to spare, while Moonblast is only a 3HKO.
- MEGA-AERO: Outspeeds and pretty much always has a move to OHKO. Aqua Tail and Iron Head are guaranteed OHKOs, EQ OHKOs after rocks, and even Stone Edge deals ~70%. It can't take a Diamond Storm and has to watch out for Rock Polish, but that's why it's a check, not a counter.
- CELEBI: Giga Drain 2HKOs, and Celebi laughs at most of Diancie's attacks.
- ENTEI: Diancie doesn't like switching in, as even Sacred Fire 2HKOs, but unless it catches a burn Diancie will outspeed and OHKO with Diamond Storm.
- HYDREIGON: Another threat that Diancie mostly craps all over, although Scarf versions can use Iron Tail or Flash Cannon to easily OHKO.
- MACHAMP: AV Champ can survive a Moonblast even after rocks to OHKO with Heavy Slam, or use Bullet Punch to deal 74.8-87.9%. CB Champ can't switch in for fear of Moonblast, but OHKOs with Bullet Punch.
- MIENSHAO: Moonblast OHKOs, but Scarfshao stands a pretty good chance of OHKOing with HJK.
- SWAMPERT: Moonblast is a 2HKO against physically defensive Pert, but uninvested Scald is a guaranteed 2HKO and EQ stands a decent chance of OHKOing. With a mixed defensive spread Swampert can even avoid the 2HKO from Moonblast.
- MEGA-ABSOL: Mostly countered by Diancie unless it packs Iron Tail or it gets to +2 (at which point Play Rough has a good chance of OHKOing after rocks).
- MEGA-AGGRON: Earth Power is a 2HKO, so it doesn't want to switch in except on a predicted Moonblast or Diamond Storm, but Heavy Slam OHKOs several times over.
- ALAKAZAM: Outspeeds, carries a Sash, and basically all of its moves 2HKO.
- MEGA-BEEDRILL: If Diancie's at full health, Bee is screwed, but it can knock off 35% with U-turn as it pivots out to a resist, or deal 75-88% with PJab.
- MEGA-BLASTOISE: Scald OHKOs, and Diancie only has a 30% chance to 2HKO.
- EMPOLEON: Scald does 70%, while the most Diancie can hope for is a 3HKO against the more common specially defensive variants. Or, Empoleon can carry Flash Cannon for an easy OHKO.
- KROOKODILE: Outsped and OHKO'd by Moonblast, although the Scarf version will OHKO with EQ.
- NIDOQUEEN: Earth Power OHKOs, while Diancie's own Earth Power maxes at 87.8%.
- MEGA-PIDGEOT: Mostly screwed. Diancie's pretty good against burds of all stripes.
- SHAYMIN: Seed Flare is an easy OHKO, although uninvested Shaymin can get 2HKO'd by Moonblast, so it has to be careful switching in.
- TENTACRUEL: Bold versions are outsped and 2HKO'd by Earth Power, although Scald hits weakened Diancie hard, and Tentacruel can avoid the 2HKO with some specially defensive investment.
- VAPOREON: Diancie 3HKOs at best, and Scald deals 77-90.4%.
- YANMEGA: It can hit Diancie hard, but it can't OHKO, and Diancie outspeeds Specs Yanmega and has an easy OHKO in return.
- MEGA-AMPHAROS: Yeah, this one is pretty much screwed also.
- AZELF: Outspeeds and 2HKOs with Zen Headbutt, although the suicidal rocks version has some serious trouble.
- CHANDELURE: Scarf Chandy deals 81-96% with Energy Ball, although it gets OHKO'd by Diamond Storm.
- CRESSELIA: Cresselia will save us all.
- DONPHAN: Why are people using Donphan? Anyhow, the AV version is only 2HKO'd by Moonblast, and OHKOs with EQ.
- FLORGES: Diamond Storm is a 3HKO against 252/232 Florges, which is I think the standard spread, so Florges can easily stall it out with Wish or Synthesis and has an easy 3HKO in return even uninvested.
- GLIGAR: Specially defensive Gligar is 3HKO'd at best by any of Diancie's moves, can recover with Roost, and 2HKOs with EQ.
- INFERNAPE: Outsped and OHKO'd.
- LUCARIO: Bullet Punch OHKOs without a boost. You should be running Bullet Punch anyhow for Aerodactyl.
- NIDOKING: Nidoqueen is better. Use Nidoqueen, please.
- RAIKOU: Outspeeds, is not OHKO'd by any of Diancie's moves (Earth Power does at most 84% to Specs Raikou and doesn't even have a guaranteed 2HKO against the AV version), and easily 2HKOs or can take off a chunk and pivot out to something with Volt Switch (and Specs Volt Switch is a 2HKO all on its own).
- ROSERADE: Survives anything that Diancie throws at it and OHKOs with Giga Drain, regaining its health in the process.
- MEGA-SCEPTILE: Outspeeds and OHKOs.
- MEGA-SHARPEDO: Outspeeds after a boost and OHKOs.
- STARMIE: The offensive version outspeeds and OHKOs. The defensive version outspeeds and 2HKOs, and none of Diancie's moves OHKO.
- MEGA-SWAMPERT: See Swampert above, except the mega version has a guaranteed OHKO and outspeeds if under rain.
- UMBREON: Diancie laughs at puny Umbreon.
So yeah, not a lot of straight up counters, but a huge number of offensive checks, and basically every playstyle has
something that can easily fit on their team and answer Diancie. I'm not saying it's not overwhelming, but I think people are freaking out on principle. Mega-Diancie is
really good, and can punch holes in most teams. It's certainly a concern when teambuilding. And I know that just because counters or checks exist doesn't mean a pokemon isn't broken. But I think people are really overrating just how unstoppable Diancie is. Yes, under ideal circumstances it can beat many of the above pokemon... but under ideal circumstances, many of the above pokemon beat it, too.
Personally I like having it in the tier, but I can understand if it's too much - I consider it about as good as MegaZam, which I also wanted to stay in, but which the UU Council decided was too much (and had some very good reasons to think so). But I do hope people actually play around with it and think critically before shouting for a ban just because it happens to be pretty good.