There are a lot of questions right now. Lots of people want to know if it is safe for them. Like most things I work with in science, its about probability and risk, not about certainty.
My understanding is that for the vast majority, it's been found to be far less risky than covid. That there are a few groups with a question mark. More because you can't test on these groups (e.g. pregnant women). Or known risks.
It is more to do with something which says 'don't'. An indicator that suggests there may be a risk, rather than a certainty of none.
I find it reassuring actually. That they struggle to identify people who shouldn't have it. Rather than the other way round.