The reason of the classaction lawsuit vs AMD is about their change of stance on their statements in but a few days going from "near zero risk" to "While we believe that AMD’s processor architectures make it difficult to exploit Variant 2, we continue to work closely with the industry on this threat. AMD will make optional microcode updates available for ryzen and epyc processors"
It's about variant 2, not variant 1 nor Meltdown. Whether or not that's ground for a lawsuit or fault is not for me to decide - that's why they go to court anyway. I'm sure we'll hear of it in here and a few years.
That aside, MS did fk many AMD users over with their OS-updates, leaving plenty with BSODS (finally fixed today..)
Intel got 4 of those lawsuits against them now apparently, AMD currently set at 2.