I'm catching up on my RSS feeds and I'm reading through Recognizing This Tire Symbol Might Save Your Life This Winter by Outside:

M+S: Like the topic of this article, a sidewall stamp. This one claims to indicate capability in mud and snow, but is actually obtained through a two-dimensional analysis of the tread pattern indicating a ratio of at least 25 percent void to lug.
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It’s easy to see why drivers assume M+S tires are safe to use in snow, but no test of any kind is required to earn the stamp, and it indicates no designed, intended, or incidental capability in winter weather.

As a person who has only driven to Tahoe in the winter once with chains, I had no idea. It's misleading to call it a "mountain and snow" tire when there's no winter testing backing the label at all.

Apparently finding a good snow tire is complicated. Even the three peak mountain snowflake symbol has a backing test, 3MPSF, is imperfect and may not be the best tire for the conditions you're driving in.