[hlopko]

I'd say the main the challenge that both Bazel and Buck have to face in the C++ land is that it's still too much work to migrate an existing cmake/autotools project to them. One nice project that tries to tackle that is https://github.com/bazelbuild/rules_foreign_cc. But yes, there are minor things that make migration from Bazel to Buck or vice versa not as smooth as it could be, and I'm confident those will be fixed.

To me the biggest added value of Bazel is the remote (build and test) execution (which will get a nice performance boost from https://github.com/bazelbuild/bazel/issues/6862 in Bazel 0.25; also mentioned in [3]).

(Your [3] doesn't compare Bazel and Buck, only Bazel with remote caching and without it, so it's not clear from it that Buck leverages caches better than Bazel).

And one nit, Bazel doesn't allow you to download anything in the loading, analysis, or execution phases ("the BUILD files"), those are completely hermetic, sandboxed, and reproducible (when compilers are). The package manager integrations happen when resolving external repositories ("the WORKSPACE file"), where non-hermetic behavior is allowed and is used e.g. to autoconfigure C++ toolchain, or download npm packages.