The dynamic dependency stuff looks very nice! It feels like a good entrypoint for systems that are "merely" wanting good build caching, and not being "so huge git falls apart" big.
My biggest gripe with Bazel is how when you're off the beaten path suddenly it feels like the ecosystem really doesn't want you to just solve problems yourself. Meanwhile in this Buck2 documentation, directly talking about adding good support for tools outsides of community-provided things.
I still am not a superfan of the awkward way that custom implementations get declared (which I think comes from needing to support super-giant projects? But it's jsut awkward) and all the naming suffers from Google-like "we cannot call them functions but must call them factories" NIH things... but at least there are clear docs.