https://blogs.sciencemag.org/pipeline/archives/2020/04/15/co... is a post from April of last year detailing the different types of vaccines, which does mention some of the issues mRNA vaccines have had in the past. I think there's a fuller discussion of mRNA somewhere on that blog, but the gist of it is "we've been trying this for about a decade and we might finally have all the kinks worked out."

Thank you, will give it a read.

My statement wasn't quite accurate. It was previous attempts at mRNA treatments that failed due to too many bad effects from the delivery mechanism.

Some companies gave up on mRNA then. Those that wanted to continue switched to trying to develop mRNA vaccines instead of mRNA treatments for non-viral conditions. Vaccines would require lower doses, which they hoped would reduce the problems with side effects.

Experimental mRNA vaccines were developed for a few viral diseases, but not had gotten through trials and been licensed for use in humans.

Some links to sources are in the history section of the Wikipedia article on RNA vaccines: https://en.wikipedia.org/wiki/RNA_vaccine#History