I don't have any statistics on that, so I can only speak from personal experience and knowledge from my own personal work.
I have been in both unionized and non-unionized environments. I have been in a unionized environment as a manager, so I didn't have the union to back me up. In the unionized environment I had a group of people who were supporting me, to whom I could go to if I had a problem, and they would fight the battle for me. In a non-unionized place and as a manager, I was on my own. The difference is not so much whether it's better to be part of a union; what it comes down to is having the networks and the people around you to fight with you.
When I was in my management position and had an issue, although I didn't have the union and I was on my own, I had a lot of people who surrounded me who were also managers and who fought the battle with me. It's probably easier if you're in unionized work because they are organized around it, whereas if it's non-unionized, you have to build that network; you have to build that understanding.
With regard to the previous question you asked, about whether there is more support if one gets the disability after employment or coming into a workplace as a person with a disability, as in my case, specifically with vision loss, people don't understand. If I go in and say I need to use the computer, they look at their own world and say that they look at the screen all day, so how is that person going to be able to use a computer effectively? How are they going to get around? Is there going to be a problem with the dog? Is the dog going to damage things in the office? There are all sorts of questions they have, and it's too much for them to think they can solve all those problems, so they go to the next person.
If you're in the job and you suddenly have a disability, you've already established your credibility. You've already established that you have knowledge, expertise, and competency in the area. I think the employer is more willing to provide that accommodation and support. The barrier there is that the person, before getting back into the workplace to accept all that support and accommodation, now needs to be supported in that pre-employment because it's like going back to the beginning. They now need to learn new skills and new technologies in order to get in and do the job differently.