WPT cover a dozen or so missing bits.
Beyond that, the clearest cut ones are cataloged and polyfilled at https://github.com/w3c/mathml-polyfills. Compare https://w3c.github.io/mathml-polyfills/acid-test.html in two tabs with and without the transform applied.
For things that are implemented but have known bugs, there is for example a layout bug with mathrm introducing whitespace in Firefox between symbols, where Chrome/Safari/MathJax don't:
