I am developer of TeX4ht, one of systems used for the LaTeX to XML conversion. The development is in progress, as can be seen from the ChangeLog. It is driven by user requests. I can say that we don't get many requests from publishers, so if they tell you about errors in the conversion, they probably don't think it is necessary to tell developers about issues they face. Errors obviously cannot be fixed without bug reports.
Other issue is that there is lot of different XML flavours, TEI, Docbook, ODT, JATS, etc. Some of them have quite strict requirements regarding document structure. I can say that it can be quite nightmare to debug interaction between LaTeX packages, TeX4ht and validation requirements of various XML formats. It is just too much time consuming to study various specifications that often don't contain examples, so it is hard to understand them often.
For example, I've recently tried to add the basic support for JATS, but it requires specific structure of the document, which doesn't correspond to the structure of usual LaTeX document. Some post-processing using XML transformation tools will be necessary. We have built-in support for such transformations in TeX4ht luckily. But it is not easy process unfortunately. Before I could complete it, I was overwhelmed by bug reports and feature requests from TeX4ht and my other projects, so I haven't found time and energy to progress in this field in the last few months.
So in conclusion:
the best solution is to convert your document to HTML5 or ODT
make4ht -f odt filename.tex
Other formats have worse support, simply because there is lack of user requests for them.
There is lot of people who want to convert their documents, but very few who actually work on tools that enable it. The problem is that the conversion is a bit difficult process.
In TeX4ht there are several separate stages in the conversion process - first we inject some code in LaTeX commands. This code is then used to insert XML tags. We patch LaTeX core and hundreds of packages. This is the hard thing, as you must understand code of the package that you try to modify, and TeX4ht mechanisms.
Easier part is the configuration of the output formats, because you don't need to be expert in TeX programming for that. Usually, you can just copy configuration for a specific command or environment from HTML configuration file and modify it to use correct XML tags.