mRNA technology is (relatively) new, but was rapidly building momentum well before covid.
In previous studies and trials it tended to work well with no unexpected side effects, in both humans and animals.
Some people confuse DNA and RNA. mDNA pharma has at least the potential to cause undesired DNA changes in the host which could be dangerous. mRNA does not, since it doesn't engage with the subject's own DNA. So no reason to think that mRNA has any particular risk of unanticipated side effects.