> > In the previous, separate repository world, this would've been four separate pull requests in four separate repositories, and with comments linking them together for posterity.

Tooling issue. Bitbucket will happily link PRs if instructed so.

> prevents a lot of mistakes from not having the right revision synced down across different repos.

Tooling issue. Depending on setup, "tooling" could be few lines in your favorite scripting language.

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Monorepo steps around some tooling issues, but sacrifices access control, checkout speed