So my cousin (age 21) appears to be suffering from myocarditis after getting vaccinated. I know two other people who have what appears to be something similar. So in my circle of acquaintances, I know of three young men who are having heart issues after getting vaccinated. Just based upon my own anecdotal experience, this appears to be a very real potential side effect of the vaccine. However, we have been told it is "extremely rare." I'm not so sure. Here's an article of a Belgian cycling team where three of their cyclists had heart issues following the vaccine. That's not even medium rare.
https://www.brusselstimes.com/belgium/181660/doctors-advise-against-intensive-sport-after-covid-vaccination/