Two bullet points jumped out at me straightway:

- Single monolithic code tree with mixed language code

- Development on head; all releases from source

Just one other concern I have now.

Something like Make, or most other build-systems run pretty much straight out of the box.

Something like Bazel lets say is apparently an order of magnitude more complicated. There's a few more moving parts.

My only remaining qualm relates to the additional cognitive load of this stuff. At scale, as in Google this isn't so much a problem because these costs would diminish at the point where you can actually assign people to look after it.

I really like the idea though! Would be nice to work in a place like Google some day ...