blaze/bazel was a big improvement over its predecessor (a set of python scripts that generated huge makefiles), but that did not make it free of accidental engineering. The google infrastructure teams were very tightly staffed, so for a long time, it was held together with proverbial duct tape and heroism.

Source: I was on part of the team that did the open sourcing, and had to clean lots of cruft in the code base that had accreted over 8 years.