Have not been playing UU much, but I decided to get reqs here. I think it's pretty clear that Mamoswine should be banned.
Used a team with the Life Orb variant and it was able to make progress each game, even if facing one of the few checks or a tech to slow its progress (i.e: Balloon/Shuca Cobalion), while being absolutely devastating sometimes. Mamoswine may not be particularly durable, but it is not overly challenging to get in safely, especially thanks to Ice Shard threatening a large portion of the faster metagame. The combination of Earthquake and Icicle Crash allows for it to break through just about everything, too. Fully defensive Milotic can potentially eat two Earthquakes from full without hazards up, but a Knock Off on the switch-in or some hazards being set can lead to this not being a serviceable answer either (especially if you run Adamant, which I personally did not). Golis can also stomach a few hits, but a crippling ability coupled with a lack of recovery makes it a very short term countermeasure. Weezing also can do the trick, but I find it fairly easy to wear out as well. Outside of these three and a few other uncommon things like Vaporeon, there is not much defensive counterplay to Mamoswine. This is a problem for a Pokemon as common and convenient to use as Mamoswine. As for the other side of the spectrum, Ice Shard allows for it to prevent revenge killing from a lot of faster Pokemon like Noivern, Flygon, chipped Celebi, Haxorus, Roserade, and Rotom-C. It is true that there have been more Inteleon sightings recently and Cobalion/Lucario can reliably revenge kill it, but that simply is not enough to keep Mamoswine fully in check. I feel like the overall lack of counterplay to Mamoswine in the metagame is alarming and it should definitely be banned.
Used a team with the Life Orb variant and it was able to make progress each game, even if facing one of the few checks or a tech to slow its progress (i.e: Balloon/Shuca Cobalion), while being absolutely devastating sometimes. Mamoswine may not be particularly durable, but it is not overly challenging to get in safely, especially thanks to Ice Shard threatening a large portion of the faster metagame. The combination of Earthquake and Icicle Crash allows for it to break through just about everything, too. Fully defensive Milotic can potentially eat two Earthquakes from full without hazards up, but a Knock Off on the switch-in or some hazards being set can lead to this not being a serviceable answer either (especially if you run Adamant, which I personally did not). Golis can also stomach a few hits, but a crippling ability coupled with a lack of recovery makes it a very short term countermeasure. Weezing also can do the trick, but I find it fairly easy to wear out as well. Outside of these three and a few other uncommon things like Vaporeon, there is not much defensive counterplay to Mamoswine. This is a problem for a Pokemon as common and convenient to use as Mamoswine. As for the other side of the spectrum, Ice Shard allows for it to prevent revenge killing from a lot of faster Pokemon like Noivern, Flygon, chipped Celebi, Haxorus, Roserade, and Rotom-C. It is true that there have been more Inteleon sightings recently and Cobalion/Lucario can reliably revenge kill it, but that simply is not enough to keep Mamoswine fully in check. I feel like the overall lack of counterplay to Mamoswine in the metagame is alarming and it should definitely be banned.