Klarna users are being signed in to random accounts

I had once contact with Klarna. It required me eight weeks to teach until they accepted the truth - I didn't owed them a cent. Just one of the usual startups around outsourcing, minimum wage and avoiding actual work.

Lesson 1: If someone want to sell you something and doesn't want make the bookkeeping itself, avoid them.

Lesson 2: In doubt? Cash only.

I can understand avoiding a company due to a bad experience but that sounds like a rather general and rather restrictive conclusion. Did you mean bookeeping specifically, or payment handler, as they are somewhat different things?

For small businesses using a payment processor removes a massive barrier to market entry. Many small business hire real world external accountants to do their bookkeeping, so "avoiding actual work", would you avoid them as well? I do some work with the accounting & invoicing teams in our corporation and there is a LOT to take into account that would cripple a startup with only a handful of employees.

Bigger companies use services like Klarna not because they can't (often they have other payment methods as well and do their own bookkeeping), it's because customers like to use them and failing to use something like Klarna means their customers will shop elsewhere.