Accessibility is not that simple, and it is not determined just by the software, but also by how the software is used by question creators, and even what is the task that is being assessed. For example, "Identify the parasite in a photograph of a microscope slide of a tissue sample" is a valid skill to learn and assess in a pathology course (e.g. with drag-drop markers question type) but is obviously not possible for someone who is blind, without very special assistive technology.
Also, there are many different types of disability, not just blindness.
Over the years, we have put a lot of work into making Moodle quizzes accessible, and the results are pretty good. For example all the question types can be answered using only the keyboard. No mouse needed. We have also tested with voice input and while I think there are a few bugs, almost everything works.
In summary, if you think about this issue while authoring your questions, then you can make quizzes that comply with the recommendations of WCAG. There are a few known bugs (search the tracker for specifics), however, I am not convinced that some of those really are bugs, or that 'fixing' them would make things better for users.
Another way to look at it is that the Open University, UK, has more disabled students than all other UK universities combined, and we use a lot of Moodle quizzes, and only occasionally do we hear that a student had a problem with the mechanics of answering the quiz.
