[simoncozens]

LuaTeX gives you hooks into the TeX system to customise certain aspects of the program's behaviour. But you can't customise the core TeX algorithms. Because the whole of SILE is written in Lua, you can replace any of the core components.

For instance, grid typesetting is a known difficult problem in TeX and its derivatives, but it's very easy in SILE because you just change the way vboxes are added to the vertical list; it's about 30 lines of code.

The frames support is pretty compelling for me as well, and will be even more so as soon as balanced frames are finished.