Can't say much about Bazel. I understand it's used at Google for their increasingly polyglot builds. I just needed to isolate the incantations to get j2cl + closure-compiler (= their migration target for gwt apps) running together in a somewhat quality-assured way such that I don't run into problems related to non-aligned versions since closure-compiler, while very powerful, has always been something of a bitch to setup and run. Ended up analyzing Bazel logs to get at the needed command-line parameters, configs, etc. Would have preferred a doc about running j2cl/c-c outside Bazel rather than bundling it, since both of these programs are still just Java command-line apps. As I understood more of Bazel, I didn't find it half bad. That says a lot since my tolerance for over-reaching build tools is near zero - a couple months ago I even did the opposite of what you're suggesting and migrated my old Java builds from maven (and ant) to straight Makefiles, for a significant win in clearness, robustness, performance, scriptability, and network traffic.
