I'm not aware of any great data on myopericarditis for very young people, and not as many have been vaccinated, so I extrapolate from older age groups:

Of 3 482 295 individuals vaccinated with BNT162b2 (Pfizer-BioNTech), 48 developed myocarditis or myopericarditis within 28 days from the vaccination date compared with unvaccinated individuals (adjusted hazard ratio 1.34 (95% confidence interval 0.90 to 2.00); absolute rate 1.4 per 100 000 vaccinated individuals within 28 days of vaccination (95% confidence interval 1.0 to 1.8)). -https://www.bmj.com/content/375/bmj-2021-068665

Among more than 2.5 million vaccinated HCO members who were 16 years of age or older, 54 cases met the criteria for myocarditis. The estimated incidence per 100,000 persons who had received at least one dose of vaccine was 2.13 cases (95% confidence interval [CI], 1.56 to 2.70). The highest incidence of myocarditis (10.69 cases per 100,000 persons; 95% CI, 6.93 to 14.46) was reported in male patients between the ages of 16 and 29 years. A total of 76% of cases of myocarditis were described as mild and 22% as intermediate; 1 case was associated with cardiogenic shock. After a median follow-up of 83 days after the onset of myocarditis, 1 patient had been readmitted to the hospital, and 1 had died of an unknown cause after discharge. -https://www.nejm.org/doi/full/10.1056/NEJMoa2110737

So vaccine-related myopericarditis looks rare and mild. Effectively, and possibly literally, no one dies as a result. I actually ended up with a probable case after my second dose, but it was extremely mild (no functional impairment, just periodic discomfort for some months). I suspect most cases are something similar- not fun, but not spooky. I'm not a doctor nor do I have any relevant specialty, but I'm reasonably sure that reducing incidence of MIS-C or other rare severe outcomes of covid is a net win in younger people.