No overlay product on the market can cause a website to become fully compliant with any existing accessibility standard and therefore cannot eliminate legal risk.
They do not correctly adress every error: accessible coding is key to ensuring that your website is inclusive to all users. For instance, missing alternative texts require a manual review to add meaningfull contents. And, overlays are not changing the source code so far.
They don't make your website fully compliant: only capable of detecting around 20-30% of all a11y issues on websites (the easy stuff). A few examples of the issues an a11y overlay cannot fix:
incorrect heading structure
unclear hyperlink text
no labels on form fields
identifying decorative images,
They can disrupt the UX for disabled users: people using screen readers will find it disruptive to their user experience if they have to switch to using the overlay assistive technology. Overlays can also bring about new problems for users, by affecting the layout. That's why some disabled users are blocking overlays.
They can make access to information worse, like Steve Faulkner said in his article of TPGI or like Haben Girma in a video about accessiBe.
They have a poor value: many disabled individuals already have the software that they require to navigate the web and can easily leverage features already built into their browsers and OS.
A more technical example, about UserWay: https://adrianroselli.com/2021/09/userway-will-get-you-sued.html