I'll add a point, because otherwise your answer was excellent:

This mechanism was once explored as a means of performing gene therapy in human beings (a couple of decades past). It was abandoned because of its utter failure to produce lasting genetic effects.

That's a big plus here: we have a well-explored platform for having the cells transiently produce a protein we want the immune system exposed to, but which we already know is useless at producing lasting effects. The only lasting effect here will be in the memory of the immune system, not in the cells affected by the shot.