None of the overlays on the market work as advertised, as I address in my 2015 post Be Wary of Add-on Accessibility (and which I still update).

The thing is, they can’t. Every industry professional knows that automated testing tools can only identify about 30% of accessibility issues. This is a function of how WCAG is written to consider content beyond just mark-up. That means even the best overlay starts at the 30% mark and then has to somehow understand author intent.

Instead, you have to start from good and accessible defaults and build from there. If your thing is already built, then you need to identify your issues and make a plan to address them. I cover some of this in my post Sub-$1,000 Web Accessibility Solution.