I'm not an expert, but my understanding is that the amount of customization in an mRNA vaccine is much less than in more traditional vaccines, so the room for unexpected problems is essentially just the protein.

This is similar to how flu vaccines vary from year to year, but the mechanisms and changes are well understood so the approval process can be streamlined, but we don't expect issues from this year's flu vaccine that we didn't see last year, even though they are different.