I concur, long time ago I was forced to use XSLT in a business interoperability setting and it was a nightmare when I needed to do any advanced (=competitive) stuff done; it ended up almost as incomprehensible as a multi-stage map-reduce algorithm.

Now for my own e-commerce ETL system I actually had to do a transformation engine encompassing JSON, XML, SQL and NoSQL in Python and rather decided to stick with the KISS principle, allowing declarative stuff for basic simple operations such as multi-key joins and offloading advanced stuff directly to customizable Python scripts.