I don't see many details about the sandboxing/hermetic build story in the docs, and in particular whether it is supported at all on Linux or Windows (the only mention in the docs is Darwin).
It's a good sign that the Conan integration PR [2] was warmly received (if not merged, yet). I would hope that the system is extensible enough to allow hooking in other dependency managers like vcpkg. Using an external PM loses some of the benefits, but it also dramatically reduces the level of effort for initial adoption. I think bazel suffered from the early difficulties integrating with other systems, although IIUC rules_foreign_cc is much better now. If I'm following the code/examples correctly, Buck2 supports C++ out of the box, but I can't quite tell if/how it would integrate with CMake or others in the way that rules_foreign_cc does.
(one of the major drawbacks of vcpkg is that it can't do parallel dependency builds [3]. If Buck2 was able to consume a vcpkg dependency tree and build it in parallel, that would be a very attractive prospect -- wishcasting here)
[1] https://buck2.build/docs/developers/windows_cheat_sheet/ [1a] https://github.com/facebook/buck2/blob/738cc398ccb9768567288... [2] https://github.com/facebook/buck2/pull/58 [3] https://github.com/microsoft/vcpkg/discussions/19129