This could be interesting to look into. It looks like there already exists a project that can generate BUILD files for external crates.

My 2 main gripes with Bazel was that it was a pain having to rewrite the build system for all of my dependencies and that it's claim of being reproducible is weak in the sense that there are not even warnings when you use resources from the base system (eg. compilers can include files from the base system which may not be the same / exist on another) (this is a problem since I wanted to use a header from the dependency I built with Bazel and not something already on my system)