Overall Buck and Bazel are quite similar as they are both converging to Starlark DSL.
However there are still some differences:
Buck is much more opinionated than Bazel.
Buck models slightly better C++ projects[1] and currently it's remote cache is more efficient[2].
Bazel is more extensible and offers also remote execution.
Bazel has a bigger community and it's roadmap is public.
There has been more than 350 C++ [4] libraries been ported to Buck for the Buckaroo Package Manager[3].
There are also technical details that manifest in some odd ways but are not significant.
There is a nice paper by Simon Peyton Jones (creator of haskell) and others that goes into the design details [5]
Uber is migrating away from Buck to Bazel. One major reason is poor support for a variety of languages (Go being one big one, where fixing issues was historically slow due to a need to upstream fixes to Buck core).
I haven't worked with Buck myself, but colleagues who evaluated it for JS have expressed concerns with lack of support/ecosystem there as well. In comparison, there are various Bazel rulesets for JS/Typescript, and I've had some pretty good experience w/ implementing rules myself. The Starlark docs are good.
Another thing going for Bazel is its ability to embed external codebases into a build system. This mechanism allows rules to be shared among repositories in a reusable fashion.
Redo is an awesome replacement for make - way better in many ways, while being much simpler.
However, it doesn’t do things buck and Bazel do such as making sure only declared files are indeed used, or tracking compiler and toolset versions on its own.
