1. Because every country's economy is not identical to Qatar's. Workers in many countries are more productive (in monetary value, for instance) compared to those in many other countries. Loosely, having workers that bring in more money would allow for needing fewer workers.
2. I don't have a strong opinion on UBI right now, but I don't see how Qatar's exploitative system somehow disqualifies UBI in general. Based on what you described, it doesn't even sound like a universal income: it's neither universal nor disposable income.
So it's too early to tell. But if it does work, then that means under some conditions it can work. If it cannot work, then, there is little hope as their system is kind of handicapped to have a better chance than most.
