I second that the hackability of using a tool written in an interpreted language is a huge advantage, but Jekyll can be darn slow and with Hugo (written in Go),

I second that the hackability of using a tool written in an interpreted language is a huge advantage, but Jekyll can be darn slow and with Hugo (written in Go), the need for plugins is actually quite low in my experience - thanks to "shortcodes", Go templating, and render hooks. If you want to customize the Markdown parser or similar, it requires to change the Go code and compile everything yourself, which is not great especially from a maintenance perspective. But I wonder, would it be possible to add more extension points and then only compile plugins written in Go instead of the full source code of the tool plus your modifications? IIRC, Go does support something akin to shared libraries but not cross-platform.