As somebody who unfortunately has a Stage IV diagnosis I have been researching mRNA and there have been promising results such as the MSK pancreatic study below, but still much to be ironed out — they had half the participants get a response but the other half nothing, even though each treatment was individually targeted and customized. They are doing a larger study now to try to see what other factors may be at play.
mRNA vaccines need a target, and if there is a target, there are several approaches that already do this (anti-body drug conjugates), and sometimes work, sometimes doesn't.
I don't think anybody thinks it "didn't work out". It's still actively ongoing:
seems like it's almost the same methodology in making the immune system target specific proteins.