The difference is that if you sue someone, they will by definition face legal action. A complaint is just a tip-off. In only a tiny minority of cases will there

The difference is that if you sue someone, they will by definition face legal action. A complaint is just a tip-off. In only a tiny minority of cases will there be any action, and most of those will be a warning letter. Admittedly these cases in the link are very likely to be investigated, but they would have probably been even without the complaint. Apart from anything, the ICO in the UK is already investigating Facebook because of Cambridge Analytica.