The mechanism for the narcolepsy was a protein present in the virus itself. So actually getting the flu would have been much worse for those with the genes that made them susceptible.
It doesn’t sound like mRNA vaccines would have this vulnerability. Though I do take your point that it’s possible something like the swine flu narcolepsy event would only be found after the fact. However that would be a pretty small consequence since in this case the flu also would have caused worse narcolepsy.
https://www.youtube.com/watch?v=4bOHYZhL0WQ
Vaccines are also very young. We've had them for 185 years, and there were probably a lot of side effects from people getting stabbed with Horse Pox, but a lot of them probably just died and we didn't collect data back then they way we do today.
It's not comparable.
E.g. I believe it was the swine flu vaccine that caused narcolepsy in a small percentage of people receiving it. But that was apparent immediately.
To my knowledge no one has identified any slow acting consequences of a vaccine that would not have been obvious from the first rollout of a vaccine.
So this question is mostly academic. Unless you’re someone in the UK slated to get the first dose, you probably won’t even have an opportunity to get the vaccine before the effects in early groups become known. And for those in high risk early groups, the risk of covid surely outweighs the risk of vaccine.
Nothing has shown up in trials so far so I’m not expecting side effects beyond the known effect of short term flu like symptoms for a couple days.