As I understand it, the big problem isn't so much chemical as physical - they need to encapsulate the mRNA evenly within lipid particles of a reasonably uniform

As I understand it, the big problem isn't so much chemical as physical - they need to encapsulate the mRNA evenly within lipid particles of a reasonably uniform size, and there's some pretty advanced microfluidic technology involved in doing this. Conventional vaccine production has the advantage that biology takes care of all that pesky encapsulation business.