This method has been in development since 1978 (when then-young Hungarian scientist Katalin Karikó started playing with mRNA in her lab in Szolnok) until today. Hundreds of mice died during the search for bugs before anyone dared to inject a human subject.

It cannot be ruled out that the whole concept of RNA vaccines still has bugs, but it is nowhere near "it compiles, ship it now!" software betas. The process is much more risk-averse.