Restoration Shaman is a healer that got relatively few changes coming into 9.2, but the biggest change they did get was the ability to wear 2 legendaries while playing Kyrian.
深震之石 +
狂暴暮钟旋涡 is an absolutely absurd amount of damage for M+ while they are also able to keep the party alive with good kit throughput.
Their damage coupled with the fact that Resto Shaman has top tier M+ utility in the form of
风剪,
电能图腾,
地缚图腾, and
妖术 makes other healers difficult to compete. Resto Shaman is generally a jack of all trades healer in raid, but this definitely works to their benefit in M+. With them having no identifiable weaknesses, they are able to really carry in M+.