I think the autoimmune process is a bit different. And it applies to any vaccine, not just mRNA vaccines. It has to do with the adjuvants. Basically your body doesn't care about a dead virus and doesn't build defenses against a dead virus. So they add adjuvants to the vaccine that irritate and enrage the immune system which then decides whatever is in the room is guilty... Usually that is the dead virus, but sometimes it might be a bit of peanut protein, or some of your own specialized (not widespread) body tissue, or whatever happens to be wandering on by at the time.

I don't think mRNA is especially worse, but anything new should be treated with skepticism. In this case (covid mRNA treatment) the spike protein anchor section failed to anchor to the cell membrane, but they travelled around the bloodstream (of at least some vaccine patients, probably via an injection that happened to be too close to a small blood vessel) and that stiffened anchor section turned out to be a real problem for other tissues such as the endothelium. At least that's how I understand it.