I have written a small article on FO render engines. It is a comparison between FOP, XEP, XSLFormatter, and PassiveTeX. It also includes a stylesheet that checks if the fo-objects or fo-properties that you use are supported by the various render engines.
You can find it at: http://www.diderottrack.nl/en.articles.xslfo.html
Hope you enjoy it. Please send any comments suggestions.