Mega Glalie is too strong for NU. As far as I can tell, only Lapras, Avalugg, Walrein, Steelix, Rotom-F, Furfrou and Regirock avoid a 2HKO.
252 Atk Refrigerate Glalie Double-Edge vs. 252 HP / 252+ Def Uxie: 144-171 (40.6 - 48.3%) -- 67.6% chance to 2HKO after Stealth Rock
252 Atk Refrigerate Glalie Double-Edge vs. 252 HP / 252+ Def Eviolite Ferroseed: 123-145 (42.1 - 49.6%) -- 30.5% chance to 2HKO after Stealth Rock
It's honestly just as bad as Dragalge considering it's not reliant on a Choice item and can actually outspeed things.
I wouldn't compare M-Glalie to Dragalge for a lot reasons: Way faster, not a wall-breaker, Physical instead of Special (although it can run special sets, physical sets are superior currently), poor coverage on Glalie, poor defensive typing, etc.
Viable M-Glalie counters include: Piloswine (Thick Fat allows it to absorb Return/Double Edge and EQ does even less), Avalugg (Avalugg.), Ferroseed (Takes 2 hits, kills w/ Gyro Ball after Iron Barbs damage), Miltank (Thick Fat allows even SpDef Miltank to completely stop M-Glalie), Regirock (Hard body baby knocking any Glalie back), Qwilfish (EQ does ~40%, can Twave and then proceed to Pain Split or try and paraflinch w/ Waterfall), Steelix/M-Steelix (Hardest body baby knocking any Glalie back), Poliwrath (Resists stab, hits for SE damage or can go for Scald), Granbull (preferably RestTalk), Regice (resists stab, fires off Tbolts), Frillish (The lord)
Viable M-Glalie checks include: Feraligatr (resists stab, fires back w/ Waterfall+Aqua Jet), Prinplup (Resists stab and can fire off Scalds), Samurott (Same deal as Feraligatr, Torkoal (takes 38% from EQ, fires back w/ Lava Plume/Yawn), Metang (EQ does 45% to SpDef, 33% to PhyDef, fires back w/ Meteor Mash+BP), Hariyama (vs 0/128 -Return does 32% -Double Edge does 38%, fires back w/ CC or BP if severely weakened (BP does 40%), Rotom-F (resists stab, Can Will-O-Wisp or Volt Switch to wear it down)(Pyroar/Typhlosion can obviously take a Return/Double Edge + Ice Shard combo, but won't like taking EQ, Gurdurr (Mach Punch does 55% and can take two Returns, Mach Punch kills after Rocks+Double Edge recoil as well)
Mons that get usage and can answer M-Glalie (I don't like these but others do ?_?)- Lapras (4x resists stab, hits back w/ Surf), Walrein [?????] (8x resists stab 252+ Atk Refrigerate Glalie Giga Impact vs. 232 HP / 220+ Def Thick Fat Walrein: 25-30 (5.9 - 7.1%) -- possibly the worst move ever), Furfrou (Double Edge does 30%)
M-Glalie obviously hits like a truck, but is hindered by a few things. The first is the obvious weakness to Stealth Rocks. If Glalie runs Double Edge, it's survivability is hindered. Glalie boasts decent bulk with 80/80/80 defenses, but an Ice typing leaving it with limited switch ins. It requires more support than other sweepers, such as Sawk, due to it's limited coverage and weakness to rocks. If the argument for banning is power, why have we not banned Sawk? He's a little slower than M-Glalie, but has far fewer switch-ins (I'm not saying ban Sawk, just an example). M-Glalie is weak to two (3 if you count Vacuum Wave) forms of priority, Bullet Punch and Mach Punch as well. Glalies coverage is "OK" in terms of hitting things neutrally, but it can't wallbreak through mons that resist it's stab and aren't hit for SE damage by EQ (besides Jynx/Cryogonal who have paper for defense).
M-Glalie also a sort of 4MSS depending on the set it wants to run. If it wants to run an all out attacking set, it's fine with Ice Shard/Return (or Double Edge)/Explosion/EQ; however, it runs into a problem when it wants to include utility moves. M-Glalie has to two great support moves in Taunt and Spikes which allows it to Defog block, hazard stack, or both. Unfortunately, this means it needs to replace 1 or 2 moves in it's arsenal. Explosion seems gimmicky, but allows it to get off a last ditch effort vs walls to weaken them for it's team. Return is it's main stab (or Double Edge) and EQ allows it to hit Steel/Fire Types whilst Ice Shard is the much needed priority move that allows it to hit faster mons such as Archeops into Defeatist, Sceptile/M-Sceptile, Scyther, Rotom-S, +1 Vivillion, Swellow, +1 M-Altaria, etc.
I've seen people argue that it's Explosion makes it broken because it gets stab. If that's the case, what's stopping CB Lickilicky who has superior coverage as well as stab Explosion. It's Explosion is indeed threatening, but it comes at a hefty cost. The life of your Mega. On top of this, it's generally predictable, since it's used as a last ditch effort, and can be mitigated by several of it's counters (ie Miltank drinking it's own Milk the same turn, Ferroseed protecting, Poliwrath resting). Explosion is indeed a very powerful weapon in M-Glalie's arsenal, and is pretty hard to play around, but at it's worst it's a 1 for 1 vs teams that lack a switch in (IE Offense which isn't meant to be taking hits anyways let alone giving m-Glalie the opportunity to get in and click Explosion).
Perhaps a core will appear further down the line that makes M-Glalie broken, but currently I think it's just a threat that needed to be accounted for just like Sawk or Typhlosion.
Freeze Dry did slip my mind during this post, but it also forces Glalie to free up a slot, most likely Ice Shard, doesn't receive a boost from Refridgerate, and is only base 70 power. It does allow it to 2hko Physdef water types however and makes it harder to wall. Assuming it gets rid of Ice Shard, it now becomes much easier to revenge kill so a trade-off has to be made. It is a very good point though, completely forgot it existed. My stance doesn't change as it still has switch ins, but Freeze Dry definitely needs to be accounted for. Also, I wouldn't say Ice/Ground coverage is good on it's own. I think Stab is needed to make it very effective (ie Piloswine/Mamoswine) and Glalie only gets Stab on it's "Ice" moves.
