FOSS is big aspect here. There aren't that many production-ready FOSS rendering engines out there. You have Cycles which is very tightly coupled to Blender, Mit

FOSS is big aspect here. There aren't that many production-ready FOSS rendering engines out there. You have Cycles which is very tightly coupled to Blender, Mitsuba which is purely academic, and LuxCore which is probably closest competitor for Appleseed. Just recently DreamWorks released their MoonRay, but it is still so new that it remains to be seen where it falls. Looking at the feature list, Appleseed seems very full-featured. One thing that struck out to me is having spectral rendering support, which is still relatively rare feature. Otherwise they seem to have strong emphasis on OSL, so that might be nice? I guess the lack of differentiation/big killer features is why Appleseed didn't catch on, which is bit of a shame considering how it still seems like solid good project.