Yeah, I'm really surprised how useless this study is. They just gave some money away and that's the end of it. That being said, I do think that it's really hard (if not impossible) to simulate how UBI would work in the real world.
https://arstechnica.com/tech-policy/2013/04/microsoft-excel-...
Not even that sometimes. Like the US attempting to re-ordering the world trade system by imposing massive tariffs. They did this even though both economic theory and empirical evidence (previous historical attempts at doing this), showed it would be disastrous.
What I think could be a very interesting on-ramp to something more UBIesque, in an environment with lots of need-based social support payments like Germany, would be an opt-in flat taxation mode for earning on the side of receiving aid: taxed high starting with the first cent earned, and in exchange those earnings are explicitly excluded from and need-based considerations or thresholds. That could greatly reduce all "not worth the hassle" considerations and uncertainties. It could be implemented as a special bank account where every incoming transaction is automatically taxed, and every payment or withdrawal certified "I'm allowed to have this without putting anything need-based in question.
What tax rate? The logical first candidate would be wherever the income tax curve tops out for high earners. If that's the highest those can be bothered with before they "stop performing", you should not be surprised if low earners would not want to perform at a higher effective tax rate either. But that tax rate should not be so low that a shift to regular taxation makes no sense in cases sufficiently permanent and wellhpaying that the administrative change isn't just overhead.