I think the sweet spot is to charge a nominal fee as opposed to making it outright free. We tried free in Seattle and it resulted in buses full of nuisance activity such as drug use, harassment, violence, toilet use, and other unpleasantries that resulted in a lot of people avoiding transit altogether.
If you have a large number of people for whom a free bus is the only reasonable shelter then that is what will happen. Most European countries deal with homelessness, drug addiction, etc., differently. So I'm not sure that the experience is transferable.
Unfortunately the majority of Seattle's homeless population actively refuses proper shelter or assistance [1], but if that's different in Spain then I can see how it would lead to different outcomes.
I believe homeless shelters have policies such as not allowing dogs, and searching for drugs/alcohol, and they aren't always very safe and full of nits/flee infestations.
The better option is to house the homeless or simply give them money. Utah enjoyed great success giving homeless houses in the 2000s.
