Joe Dolson has a really information post about why WordPress accessibility is often misunderstood. In short, it's because the front-end is incredibly variable depending on the theme and plugins installed.
WordPress itself, with any generated output and the admin, are pretty good accessibility-wise. Joe summarizes his post like this, though I do recommend reading it all:
If you’re talking about the front-end experience, WordPress core truly has very little direct impact on the accessibility of your site. It’s certainly not none — there are core widgets and there is some core HTML output in areas like comment forms and menus, but almost all of that is just straightforward, traditional HTML. If HTML is written correctly (as in the WordPress core output), then the problem isn’t with WordPress – it’s with whatever CSS, JS, or error-prone HTMLis added by other resources.
WordPress is amazingly extendable – and while that makes for an ecosystem that’s vibrant and thriving, it comes with significant risks, as well.