mRNA vaccines have been well studied - from a scientific standpoint it's hard to imagine how it even could cause problems.
Here is an example of a long term problem caused by a (traditional, non-mrna) vaccine
https://www.cdc.gov/vaccinesafety/concerns/history/narcoleps...
As far as I know there hasn't been much research into the long term effects of many vaccines. After decades of availability if they were causing issues I hope somebody would have found a connection if they were causing issues. I would very much like to see research into this though.
I do agree mrna vaccines are unlikely to cause long term issues.
Just to be clear. I am very much in favor of vaccines. I just don't like being required to take it so soon after it was created.