I care a ton about fast and accurate build systems but one issue I think we haven't solved is that: people do not want to use other build tools for their language. "Why isn't it Cargo? Why not use NPM/Yarn? Why not use Pip? Why not CMake?" These questions are often rhetorical because they do not care about build systems. They don't care about the design. They don't care if your CI could be 5x faster. You will never make them care. It's good enough. You must have absolutely zero externalized cost (and therefore put in a lot of effort) to get over this hurdle. There's seemingly no way around it.

The reason a lot of people like Bazel is, I think, tools like Gazelle -- which reduce that whole problem back to "Run gazelle" and all the crap is taken care of for you. Dependencies, BUILD files, etc. People constantly talk about the "complexity" aspect, but very few people appreciate how complex Cargo, NPM, Yarn, Cabal, Dune, internally are. Because they just run "build", and it works. Bazel, Buck2, Mill, etc will all have this problem unless huge effort is put in.

TBH, this is one of the reasons why I think Nix has wildly succeeded in the past few years while more fine-grained and scalable systems have had adoption problems -- despite its numerous, numerous flaws. You get to Bring-Your-Own-Build-System, and Nix along with the blood of 10,000 upstream contributors keeps the juice flowing, and it's cached and hermetic so you see real savings. That greatly eases people into it. So they adopt it at all points on the curve (small, medium, huge projects), because it works with what they have at all those points. That makes them willing to get deeper and use the tool more.