> Stability - We've been caught too many times by changes that broke various parts of the wiki.
This is rather a chicken or egg problem. Stability will only happen once wikia stops all development of the portable infobox.
> Multiple entities per page - Some wikis use switch infoboxes, of varying complexity. Some of them just swap images, other have a more complex setup.
I think this is the case of wanting to display too much content on a small area. Anyway, Collapsing basically allows this. The only thing missing is different layouts for "collapsed" content.
>Custom attributes and classes - It's reasonable to want to have a stable API for everyone to use. Extending this to cover use of attributes within XML is equally understandable. What's not understandable, is why the ability to add arbitrary classes has been removed.
This is a valid concern, although FINALLY the templatestyles (see mw:Extension:Templatestyles) seem to be close to concluding their development (possibly by the end of the year or next year), and this may reduce / eliminate the need for any specific inline css if wikia finds it a good idea to deploy this.