The biggest issue in terms of safety with previous attempts at mRNA vaccines has been anaphalyxsis. They've worked really hard to make that incredibly uncommon - with the covid vaccines it's now much less common than it is with most antibiotics.
The other factor that has been an issue with previous attempts is them not working. That has been the case with a couple of mRNA vaccines trialled for covid - certainly not the case with Pfizer or Moderna.
