I don't have a lot of experience against or with Mamoswine, but think it should be unbanned!
The problem is against offense that it's too easily pressured. Pretty much everything on offense outspeed it and most things can KO it pretty easily. The only thing on offense it can beat are Electric types that aren't Thundurus with Superpower/Focus Blast, but those aren't too common. Otherwise it's weaknesses are too common and its bulk too mediocre to do much work against offensive. I realize it's still a good Pokemon and not as terrible as it sounds here, as it might still KO a Pokemon, but it's not broken. It can take a hit at times, but due to the fact it has no recovery and meh bulk, it will only take one and get only one shot. It can pressure offensive with Ice Shard, yes, but a lot of things resist it (Think Victini, Lucario), and the most common things it hits Super-Effectively are Flying types, which are usually Gale Wings and both Staraptor and Braviary have a chance to outspeed it. Also it doesn't even OHKO them with a Band, so it's basically sacrificing yourself if they don't have prior damage. Also you can't switch in, so have to be brought in using a double switch or after a kill.
Where Mamoswine shines is against stall. It does have very good STAB Coverage and Knock Off for Doublade and other Eviolite Users / Psychic and Ghost types. That said, I think Stall just has to adapt a little. It might be difficult to deal with, but honestly it's beatable. During the Weavile era I used Thick Fat Skarmory and it was pretty great at countering it, and so should it be good against Mamoswine. Now it can even run Delta Stream if it prefers Rock resistance and Electric neutrality over Fire neutrality. You might think: "But, isn't that overcentralizing?" or: "Aren't there better sets?". I don't think it's overcentralizing. It requires Stall teams to pack a specific counter, but doesn't it have to do that against all stall or wallbreakers (Think Victini, ErupTran, Tinted Lens Mons). The fact that it beats some stall mons doesn't make it broken if you teambuild well. Yes it can break Intimidate Hippowdon, but wouldn't Hippowdon be broken if it couldn't be broken? Hippowdon can even beat Mamoswine if it's dumb enough to lock itself into Icicle Crash if it takes Thick Fat itself (With which it can do amazingly against Victini too!) and Earthquake is easy to pivot out and still only does 65% maximally. Finally, some unorthodox sets with Thick Fat, Delta Stream or Levitate can do great work against Mamoswine, so I don't feel it should be banned. An example of something cool that could beat it is Levitate Infernape. It doesn't have amazing bulk, but can check a lot of Pokemon with its access to a quick WoW and Slack Off. It also requires a Choice Band to wallbreak successfully, as otherwise it just lacks the needed power to break through some walls.
I still think it's a good Pokemon. It has great stats and a good STAB combination. Access to priority is great as well, but it's not overpowered or broken and shouldn't be banned. Notice that I onlyt alked about Adaptability here. Refrigerate has similar counters but exchanges extremely strong Ground STAB for (I think, don't kill or quote me) a little stronger Ice STAB. It could also run Mold Breaker to get past its normal counters, but it's able to get beaten more easily that way due to way weaker attacks. Some other sets exist as well but are not nearly as powerful.
