Very few developers, not the latest version of epub, and not too skilled/interested devs.
For example: I once filed a bug report with epubcheck regarding incorrect errors after svg tags including how to recreate the bug with a simple example, and then even deciphered their java code and proposed a simple java fix and nobody did anything with it for over a year or more. It was a very simple fix and immediately obvious (i.e. need to detect end svg elements). The developers either did not understand their own code or simply did not give a damn.
Your mileage may vary.
For the record ...
https://github.com/IDPF/epubcheck/issues/680
Mar 16, 2016 -> June 17, 2017