Further, they enable a "pay later by invoice" checkout flow, again by just knowing someone's SSN. Scammers use this to order items from web stores to automated pick-up lockers with someone's else's SSN for payment info. The victim usually only becomes aware about this activity when they start getting debt collection notices for unpaid invoices from multiple stores for thousands and thousands of euros. The debt collection process in Finland is famously unfair and harsh towards the supposed "debtor" (here: victim of fraud).
Unless the "debtor" (victim) actively opposes each and every individual collection, the cases will eventually end up in court with summary judgement. This will ruin the victim's credit rating, which has devastating results for just about all aspects of life. People are known to have collapsed under the burden of all this and ended up taking their own life.
Klarna's response to all this is that they want convenient checkout experience and some fraud is unavoidable. Although there are excellent technical means available to strongly identify users in Finland, they add a minor layer of inconvenience compared to just typing in your SSN. This is OK for Klarna since they give exactly zero fucks about security as long as they can make a little buck from it.