Had a look at Charis SIL

[Vasile Gaburici]

It uses *lots* of multiple (ligature-type) substitutions, sprinkled
with some context-based substitutions, and some single substitutions
in multiple ccmp tables (some tables are class-based, some glyph
based). It's unlike any of the simple stuff that Adobe or other fonts
do. I wonder how they maintain all that... Does anyone know if they
have their own production tools?