>the participants are usually aware/suspect that this bonus income probably won't last forever

If a country fully implemented permanent UBI today, all of this would still be true. Controversial political decisions are likely to face constant challenge, and this especially applies to those decision which do not benefit the most powerful.

Being critical of this as an imperfect experiment is an odd take to me. This is what reality looks like. Having a real-world test like this is an important stage of the process of implementing UBI.