Historically, most vaccine problems show up early. First, there's the risk of an immediate allergic reaction, which, if it's going to happen, happens within minutes to hours.
Then there's the risk of an inactivated or weakened virus vaccine causing the disease it's supposed to prevent. That happened in a big way in 1955, with a defective batch of polio vaccine. The mRNA vaccines can't do that - they don't have an entire virus.
There's the Guillain-Barré syndrome with some vaccines, including the Janssen COVID-19 vaccine. If it shows, it does so in the first two months.
This would be an excellent point IF these were like other vaccines (and in fact wouldn't even fall under that label without the change to the definition made last year). As it stands this is more a rhetorical argument you've made, which does nothing to address the actual unknowns and long term concerns, but merely appears as though it does if you pretend these treatments can be represented by broad aggregated tendencies regarding "vaccines."
Then there's the risk of an inactivated or weakened virus vaccine causing the disease it's supposed to prevent. That happened in a big way in 1955, with a defective batch of polio vaccine. The mRNA vaccines can't do that - they don't have an entire virus.
There's the Guillain-Barré syndrome with some vaccines, including the Janssen COVID-19 vaccine. If it shows, it does so in the first two months.