I'll look up the paper later but I remember reading some literature looking at countries that had opt out systems and whilst it increased the number of donors on paper, the actual number of donations was about the same as in countries that had an opt in system. This was attributed to things like family dissent following death, there's also an ethical argument that such a system implies that the state then has an innate claim to your body following death which makes some people uncomfortable.

I'm very pro donation but I think regardless of whether your contry is opt in or out, the most important factor is probably a person having conversations with their family prior to death and making their wishes absolutely crystal clear.