The big problem with Sweden's approach, is that the duration of immunity is entirely unknown when it comes to SARS-CoV-2. We don't yet know what antibody level is required to generate that hypothetical immunity, including whether asymptomatic and mild-case persons (which will be the vast majority of Sweden's herd) are going to have the required level of antibodies.

They're taking a swing in the dark, with the potential risk of a very high per capita mortality rate. If the duration of immunity is short, they could kill a lot of people for little gain. They're currently running at seven times the per capita mortality rate of Finland.