The tagging work is still highly experimental. A major missing element is equation tagging: for now, you need to produce an 'associated MathML' file externally, for instance using LaTeXML. Even then, PDF readers do not support the MathML tags yet! If anything, I am sure that the LaTeX3 team would appreciate you posting minimal examples of mistagged PDFs.

If you want to produce accessible documents from LaTeX, you should convert to HTML. ATs such as screen readers just work miles better than with PDFs, and given the resources put in developing browsers compared to PDF readers, I don't think this will change any time soon. Luckily conversion from LaTeX to HTML is very feasible today, as proved by arXiv. (Shameless plug: I maintain BookML specifically to help lecturers with the LaTeX to HTML work)