Well, after reading the manual, it looks extremely promising and already usable, including to do complex things such as side notes or parallel, two-column bilingual texts.
Also, the frame system (to define a layout model for pages) looks both usable and powerful. Minor changes of the output aspect (fonts, page size, margins) seem easy to do.
Of course I would need to actually use it now, in order to have a more informed opinion and see whether the typesetting is actually good.
Good language support too, including hyphenation and (although I haven't verified that) typesetting conventions.
On the negative side, SILE has been designed at first for humanities, and as such it does not support yet mathematics, nor tables or programmable graphics (like with Tikz in LaTeX).
And of course it doesn't have the ton community-written packages that TeX has.
LaTeX is indeed quite old and somewhat painful to write, but it's package ecosystem is very rich and I suppose most maintainers wouldn't care too much to migrate to a very different platform.
Anyway, it's promising to see that people are actually working on new typesetting tools.