[barrkel]

I think monorepo makes it easier to update downstream dependencies atomically as part of an upstream change, and thus encourages a culture of unstable APIs.

That is, careful evolution of internal APIs is not given much weight, so modularity - in the sense of containing change - suffers.

I don't think monorepos must necessarily go this way, but expressing dependencies in terms of build targets rather than versioned artifacts has a strong gravitational effect. Change flows through the codebase quickly. That has upsides - killing off legacy dependencies more quickly - and downsides, wanting to delete code that isn't pulling its weight because of the labour it is inducing.

[I currently work at Google but I've only been here a few weeks. I certainly don't speak for the company.]

[raiflip]

I think this is absolutely it. A lot of best practices go down the drain when the compiler compiles the bytecode. As such a lot of best practices are about people, not the computer. APIs can be just as stable behind a network or a library, but way more people are onboard with never break your APIs than they are never break your function.