It's a technical concern. Publishing will only cause fear and doubt.
In any case, it's a matter of weighing risks. Even if there's a 10⁻⁶ probability of complications, giving the vaccine and stopping serious illness/deaths is still a better decision overall. Even if it means hundreds of cases of complications across Europe. And then, the complications might turn out to be statistically difficult to discern from uniform random anyway.
A probability of 10⁻⁶ is difficult to conceptualize so if it is published with scary words, people might interpret it as "will probably happen to me". Then if that affects vaccine roll-out negatively, publishing might be a bad decision.
Millions of people in the UK have received the AZ vaccine, which has demonstrated safety and efficacy.
The efficacy metric means protection against infection, which is not complete. But as far as I've heard, not one person who has received either Pfizer or AZ has become seriously ill or died. In other words, it "flattens the curve" instantly.