You are probably going to get a lot of people who use this thread as an opportunity to blow off steam about how they hate their boss. That's a favorite pasttime. So, remember that as you read the other posts. People complain loud and clear whenever something does not go the way the would like it to go. They pretty much don't notice it when things are going smoothly -- and their boss is supporting them in a hundred different little ways.
As you said, there are good days and bad days, good bosses and bad bosses, etc. However, the general rule of thumb is that a hospital does not want to get sued -- and they don't want to lose a lot of staff or get into additional trouble by victimizing an employee who has made a mistake. There is no financial incentive for a hospital to NOT support an employee when a mistake is made that could lead to a lawsuit. So, they are usualy "on your side" for the big things. In an actual law suit, they don't dare NOT support their staff in most cases.
However, they are "on your case" frequently for little errors, sloppy practices, etc. that could lead to big problems if not prevented. Patient safety is one of the top priorities and if your practice does not meet their standards, they will insisit you improve. That means following all procedures to the letter, documenting everything in triplicate, etc. Some people perceive that as "non-supportive," especially when staff shortages, poor working conditions, etc. make it virtually impossible to do everything in exactly the approved way.
While your question is legitimate, I think it would get predominantly negative answers no matter what profession you were investigating. That's just human nature.