[perplexed]... the UBI is bad for them because someone who's working bellow the minimum wage in his profession now has the option to say "No, I am not going to work under those conditions" ?
That option comes in the form of a perpetual free gift; made by permanently shaving off a bit from everyone else (who are likely themselves ineligible). If you try standing in line at the store behind someone who is spending their cash on vice, and using welfare to get their food for free, then you might understand.
Why should anyone be forced in a situation where they have to take every job no matter the conditions?
When an animal is hungry, they eat what food they can find. When a person needs money, they take whatever job they can get; it's temporary. Not every employer can afford (nor finds it worth) paying above a certain amount for specific work. For ANY job, it is the employer who must find it worth paying; else there is no job.
*Why is this part always so hard for non-employers to understand? Employers don't exist to employ, they employ because it's the path of least resistance, and when it becomes too much of a hassle to employ, it stops being worth the trouble, and the job goes away.
...in many low wage professions, like catering, room services, harvest hand the like, the real wage is closer to 3-5 euros per hour?
Many jobs are simply not worth that much. Many of these jobs are part time, and some people work more than one of these jobs for a combined income.
Service industry is a special case. Consider the bellhop. The job is to carry a bag upstairs. Consider the valet; the job is to park a guest's car. Consider the job of taking a drink order.
These jobs pay low wages, but come with the benefit gratuity/tips; this makes the job more valuable than the wage, because the person might make double/triple/quadruple the wage in tips. Some waitstaff might make $300 a night, above their wage,. This isn't random; it depends on the person, their skill, and demeanor. Every customer can leave a tip. If all of the waitstaff suddenly had their official wage doubled, then many of them would lose their opportunity to do the job, because many of those jobs (or their number of hours) would vanish.