Sinovac/Sinopharm are just inactivated virus vaccines, like flu vaccines. Kill the virus, inject it, stimulate antibodies. Safe dosage range can be determined through tests in animals and humans.
mRNA are a totally different beast altogether: 1. you can't predict where the mRNA will go after injecting it; 2. you can't predict how much of it will be absorbed by the cells; 3. you can't predict how much spike proteins will be produced by the cell machinery after the mRNA enters; 4. you can't tell if the mRNA will actually alter your DNA.
If the mRNA vaccine is taken up primarily in the heart cells, and large amounts of spike proteins are released in the coronary vessels, you end up with blood clots and a coronary attack and sudden death. If the brain takes up most of the mRNA, you get a stroke.