The first thing I want to know about a typesetting system is: does it do microtype? And are there any innovations at that level? I don't see any mention of this in the "SILE vs TeX" section of the manual, but to me it seems like the most important thing I'd need to know up front. After all, if I'm not going to put in the effort to make a document look really good, why bother with a typesetting system at all?

Practically speaking, I think if they wanted users to adopt this, the best thing they could do would be to write an output format for Pandoc or similar. I could try this today if such a mode were available.