Yes there is a big risk that the vaccines won't work if the viral spike protein has sufficiently diverged from the one encoded by the vaccine's mRNA template.

Yes there is a big risk that the vaccines won't work if the viral spike protein has sufficiently diverged from the one encoded by the vaccine's mRNA template. Viral proteins are constantly changing. They accumulate sequence mutations both naturally and due to selective pressure. There's a limited landscape of acceptable changes but we don't know what those are and how they relate to each other. All we can do is generate new mRNA templates to the more popular strains and revaccinate. This means that we will most likely we will end up taking a cocktail of mRNA vaccines and yearly boosters.. with new mRNA templates. "Won't work" in this case is defined on a spectrum. Moderna's vaccine is currently about 90% efficacious. Newer variants could reduce that down to say 60% or more. At some point the current vaccine could become useless.