The point is not autonomy. The point is that it keeps you alive, so that you know that if you leave your job, at least you'll still have health care and food, so you won't die by being jobless.

It sets a minimum bar.

Realistically you need both -- a UBI and a union. The UBI sets the minimum bar that all jobs must reach, the Union sets the livable bar for one particular workplace (or maybe a group of similar workplaces).

UBI and Unions serve different purposes. A UBI prevents modern slavery, a Union provides for a decent quality of life.