Are questions specifically about external render engines (engines not bundled with official blender) allowed?
For example, What's going wrong with my Luxrender Prism?
I don't see any reason not to allow these, but I suggest that it be limited to render engines with blender integration (i.e. usable directly from blender's UI).
Using external render engines seems like a fairly common thing among blender users, and I think it definitely fits under our current scope:
The Blender Stack Exchange site is for people who use Blender to create 3D graphics, animations, or games.