They don't reduce complexity. They translate C++ (static complexity) to JS (dynamic complexity).

Also it is not complexity if XSLT lives in a third-party library with a well defined interface.

Thei problem is control. They gain control in 2 ways. They will get more involved in xml code base and the bad actors run in the JS sandbox.

That is why we have standards though. To relinquish control through interoperability.