Myocarditis is a very rare side effect, you generally can't see those in phase 3 trials. You need a lot of people for these to show up, simply because they are so rare. That's nothing new here, this applies to every new drug or vaccine. And Biontech/Pfizer doesn't have the highest myocarditis rate, that is for Moderna.
The vaccines also simply work. They worked very well against the wild type where they also prevented infection, and not only drastically reduced death and hospitalisation. They still reduce death and hospitalisation against Omicron. We figured out now that you really want a third dose for best protection, but that is simply something that you can't know if you have to develop a vaccine on such a compressed timescale.
The mRNA vaccines are the best ones against COVID. AZ and J&J have more serious rare side effects, and they are simply less effective. They're still dramatically better than no vaccine, but once the vaccines were not scarce anymore in the developed world, it made sense to focus on the better ones.
As a disclaimer, I have ongoing cardiac side effects. Several doctors from general practice and in cardiology have separately told me that they all generally accept the published numbers much lower than what they see first hand. In a previous comment I hypothesised this is because the reporting systems are not good.
The vaccines also simply work. They worked very well against the wild type where they also prevented infection, and not only drastically reduced death and hospitalisation. They still reduce death and hospitalisation against Omicron. We figured out now that you really want a third dose for best protection, but that is simply something that you can't know if you have to develop a vaccine on such a compressed timescale.
The mRNA vaccines are the best ones against COVID. AZ and J&J have more serious rare side effects, and they are simply less effective. They're still dramatically better than no vaccine, but once the vaccines were not scarce anymore in the developed world, it made sense to focus on the better ones.