I took a look at the adenovirus to see how that worked. Its uses DNA rather than mRNA. Here's a good discussion:

https://www.nature.com/articles/s41541-021-00356-x

DNA has an issue, in general, since it could end up being incorporated into your DNA. That's what OptionBase1 was concerned about near the beginning of this thread. As the article notes, that almost never happens with adenovirus. That 'almost' should give you pause. After all, with hundreds of millions of doses, a rare event becomes virtually certain. Still, that might not cause harm.

However, DNA doesn't do anything on its own. It gets unzipped and a strand of mRNA is created, which then gets to the ribosome to form the spike protein. In other words, the adenovirus vaccine is essentially an mRNA vaccine precursor. Instead of starting with the mRNA, it starts with the DNA that is translated into the mRNA. It does seem like it has a chance to be more effective, because the DNA might stick around cranking out mRNA for a very long time. Heck, if it got incorporated into your DNA, it could stick around cranking out mRNA for the rest of your life.

Still, the bottom line is that the immunization mechanism is the same: Create spike protein and show it to the immune system. Also, create spike protein by using the cellular ribosomes to translate mRNA into the amino acid sequence that makes up the spike protein.