For what it's worth, they've tested this exact technology in animals for years and in people since MERS back in the early 2010s, neither of which had long term problems, and the spot on the virus they're targeting was identified in the late 00s based off of SARS research, so none of this is really new.
Foreign mRNA is removed from your body really efficiently by your immune system because that's what it was "designed" for, and if there were any significant long term issues we'd have seen them in the Phase 1/2 trial people by now!
J&J is really similar to the Oxford vaccine (both adenovirus vectors); the closest COVID-19 vaccine to ones already on the market will be the NovaVax vaccine which uses a mixture of proteins and fats to make what are called "virus-like particles," very similar to the Gardasil HPV vaccine.
Overall I don't think any of them will be substantially less safe than the others, the only effects that we've seen so far that are fairly rare and are mainly related to pre-existing allergies.
FWIW, my day job is an industrial chemist to supply pharmaceutical companies and I have a background in biochemistry and pharma; I'm confident enough in the vaccines that I signed up for the trials and received a shot as did my infectious disease specialist father and my chemist brother.
If anyone has any questions or concerns about how the vaccines work feel free to message me and I'll answer as much as I can