1. If you apply for a loan, this affects your credit rating. Yes - not get a loan, or default on a loan, simply by applying for a loan, your SCHUFA can be affected negatively. Especially if you are shopping around for loans, this is infuriating. This happens because your SCHUFA is affected simply by another entity checking your SCHUFA score. This is also really annoying because it can be very hard to get a loan as a recent immigrant to Germany, so you inevitably end up applying for multiple ones.
2. Your SCHUFA score is also affected by where you live, and how many jobs you've had in X years, and how often you move (which is often as a recent-ish immigrant).
3. Services like Klarna affect your SCHUFA - even if you have a perfect payback time.