One of my issues with it is that 'religion' can mean whatever you want it to mean. There are absolutely zero, to my knowledge, religions whose founders or holy texts preached against vaccination. When did Christ or Mohamed say vaccinations were outlawed, exactly? And millions of believers take vaccines- but then some of them also claim that it's 'against their religion'. How is it against your religion, when none of the founders stated this, and the majority of people in your own religion are using vaccines without issue? Why not say that paying taxes or stop signs are against your religion next?
Exemptions allow bad faith claims that one's personal preferences are their 'religion'. They should be constitutionally outlawed