I am not expecting a "children of men" distopy, and I'd rather have an mRNA vaccine than any other, but it is possible there might be some unexpected issues in the long run.
As to track record, the first viral vector vaccines in widespread use (i.e. the COVID vaccines) will have killed hundreds (possibly even thousands) of people via blood clots. That’s a worthwhile trade off for a disease as deadly as COVID. But it would be less acceptable against less dangerous diseases, particularly if safer vaccine technologies are available.
The oral polio vaccine "Sabin" has "live" virus. It's stronger than the injectable polio vaccine, but there is a small risk that the virus can escape and mutate and after a year or so cause polio to another person. So countries without polio cases, only use the injectable version "Salk" that does not contain "live" virus.
The first smallpox vaccine was just a similar virus for cows. The latest smallpox vaccine also used a "live" virus.
There have been several rounds of trials of adenovirus vectors vaccines, but they never went forward to widespread use.
Viral vector vaccines are a less new concept than mRNA vaccines, so it might be easy to see one as new and the other as traditional, but they're both new.
Except that it is, because it has people spooked. Humans, myself included, are just terrible at gauging relative risk, and it doesn’t really matter that you are way more likely to die in your car driving to get the vaccine than from any vaccine complication. You are putting this in your arm, and people find it scary when even the tiniest issues crop up.
(I’ll snark a bit here: wonder how many people who are scared of vaccines smoke cigarettes…because, in terms of risk…hoo boy.)
Except there are two other vaccines with no risk of bloot clots. So if you can choose, why not choose the ones that aren't linked to that?
Ofc, that mostly goes for the US and EU who have secured supply.
Where I am employers are legally obligated to give a day off for vaccination...
Beyond that, there are people who are unlikely to come back for a second dose - homeless people, those with drug problems, etc. From a public health perspective, getting those people an effective vaccine is very important because they're generally at higher risk of getting infected and transmitting it to others, in addition to generally poorer health.