At the same time the vaccines can't control which cells get the mRNA delivered as opposed to non-syntetic cells, where the cells are differentiated by their methylation and other epigenetics and gene expression control pathways, which I guess can be problematic for some cell types.
But I was making a different point. Even before you get to the final expression of the protein, there are various steps that need to go smoothly. The mRNA sequence is carefully engineered to dodge the body's various defenses, among other things - and this sort of engineering is not trivial - which is the point I thought you were also making with - "and the hardest part of developing new mRNA vaccines is to research the genetic code to put in it. ".
But maybe I misunderstood your original point?
While we're all cheering for its success, its important to remember that the current authorization was under the 'emergency use' clause. Also, our body continues to evolve to fight attempts by external agents to inject foreign genetic material into our cells. Whether the same platform with work on future vaccines will be known only by performing science. But as always, data >> opinion.
You seem well read so you've already identified one of the key problems - the genetic code that forms the basis of the vaccine. Its not really my area, but from what I understand, dealing with synthetic RNA (RNA Mimics) is tricky since its never identical to cellular RNA. All of the associated problems will already be known to anyone working in that field, so there's nothing of value that I can add anyway.