Having read through the the docs a little, my gut reaction is that they might be a little too much in love with the idea of having created a language that can serve both as schema definition and as minimal values carrier. It smells of unexpected failure modes through overuse [1].

But perhaps this is exactly the core feature: everybody who adds pkl to their software implicitly signs up for participation in whatever configuration monstrosity the downstream stack will end up having. Based on the assumption that that it will be a monstrosity anyways, and that a uniform system would be less bad than an unstructured mess.

Next stage of concerns: if it's a heterogeneous stack that shares one configuration graph, the runtime implementation that is linked into parts of the stack can't ever be allowed to evolve. Ouch. And then there's runtime performance, I wonder if that could eventually become a factor, e.g. if short-lived go processes are involved?

It all seems surprisingly ambitious, very far from the "why not json-with-comments" that was my first reaction (shared with many I assume)

[1] digression: e.g. it reminds me of how in the dark days of peak XML java, a lot of typechecking was effectively thrown overboard because for some unfathomable reason everybody agreed that it would be nice to rearrange objects without a helpful compiler in the room