It would be possible to make Angkor work with RsForm, if someone created a plugin for it. At the moment, we have only created one for Ajax Register, which seems to be one of the popular extensions to create custom user registration forms. So, there is still an alternative solution.
I have to make the same remark for Chronoforms, Breezingforms, and Fabrik. It would require to create a plugin for each. There are at least 50 popular joomla extensions that could use an Angkor plugin. However, I cannot build, maintain and support all of the these, all by myself ...