Some obstacles:
1. Building with QT5 MOC & UI files. There is a great library[0] for it but it has hardcoded paths to the QT binaries and header files assuming a system-wide installation. I had to patch the rule to point to our QT location. Then it worked fine.
2. There is no rule to build a fully static library[1]. Since we were shipping a static library to clients via our Makefile system, that was somewhat annoying.
3. We were using system links like `$PROJECT_ROOT/links/GCC/vX.Y.Z/ -> /opt/gcc/...` to point to all the build tools, but these didn't work in Bazel I think because it required absolute paths for any binaries it calls. We ended up putting them in a .bazelrc but we would need a different one for Windows and Linux.
4. Not good integration with IDEs
5. (edit) The Bazel toolchain system is confusing and I couldn't understand it after reading all its docs
Ultimately we did not keep using Bazel because we were building Python binaries and py_binary was too slow on Windows. And we didn't have enough time to write a PyInstaller rule.