MRNA isn't particularly stable and breaks down quickly. To contain it long enough, mRNA vaccines enclose it in lipids. Lipid breaks, down, body reacts to the protein.
The question is really what components of the vaccine are novel compared to other vaccines and medication. Novel components are the open question, but the effect of others should be well known.
Here's a link to the ingredients:
What are the ingredients of Pfizer’s covid-19 vaccine?
Now, the mRNA doesn't hang around in the body long. But the spike protein is found on the virus, and the immune system's reaction to that should be identical to the virus itself. There seems to be an issue of the amount in the vaccine producing a stronger response. But since it's based on spike protein, it's the same thing the body encounters on the entire virus.
The break-down of the lipids in the vaccine is thought to be understood, That said, I couldn't find where the principle lipid, ALC-0315, is used in other medications. Polyethylene glycol, AKA PEG, shows up in a lot of things. I seem to recall PEG being in a certain soft drink, but it's not on the current ingredient list.
The question then seems to be whether there's a long term effect with the lipids. If there is with the spike protein, since the body's going to encounter that anyway, that seems to be a wash.
Whether you're more likely to contract the virus N months after the vaccine is a question. Given the emergency, special emergency approval was rushed through. The average clinical trial for a vaccine takes five to ten years, without emergency approval, it's likely that it would only now be available.
What I do know is that in my demographic, there was a significant decrease in mortality between those vaccinated and those who weren't. This was prior to Omicron.