MaxPlus is a half done wrapper for the Max SDK. It is pretty terrible, but can be really good in the few areas that the implementation is complete.
pymxs is the almost 100% complete wrapper for MaxScript. Super useful!
The two modules are not really compatible as they use different objects ( SDK vs MaxScript objects ), and any benefit of using both is lost when having to convert the data types.
When pymxs was implemented in 2016, there was a full integration of a python27 interpreter ( as far as we are concerned ), which allows for python to be executed from within max, rather than having to import a python file as was the case with the MaxPlus implementation from 2014 - 2015.
Currently, a tool I have written is able to control Max instance A from Max instance B, using the win32 module. - If this is what you were asking?
Python and xml do not really play nicely together, especially the format that AD uses in Max ( .xaf ).
Though, for any native tool that uses said format, there should be an interface to modify it through, rather than editing the xml file directly.
Ultimately, it comes down to the following choices:
Are you wanting to modernise an existing advanced pipeline or create a new one from scratch? - Use Python
Do you want to extend your functions beyond the native capabilities of max, ie using numpy, scipy, json? - Use Python
If you are just writing a handful of small scripts to assist you in your day to day work? - Stick to MaxScript