You usually run into issues if you are trying to use off the shelf tools and git providers. IMO GitHub and GitHub actions sucks hard for monorepo. The fact that all actions have to be stored in a single directory for example almost certainly is unmanageable rats nest waiting to happen at any sufficiently large business with a sufficiently complex product or set of products.

This is why companies like google run their own forms of git with custom wrappers to let you do things like pull a segment of the terabyte sized repo or run partial builds with tooling that basically runs some kind of graph against the changes. Bazel for example had to be invented to help solve that problem at Google and pants similarly for twitter (who also has a monorepo)

If you are willing to invest in using tools like bazel and own building all these complex wrappers then it can be fine. But if you want to off the shelf gitlab or GitHub actions and use your IDEs built in git tooling it’s not going to be for you. That’s the difference between what’s possible or a good idea at a medium shop vs a company with 40k engineers

In my experience at a company that just moved away from monorepo, half the off the shelf vendors and foss tools out there balk at you if you expect monorepo support. We moved away specifically because at our current company size it is more tolerable to have our different products separate and eat the occasional pain of mass pattern adjustments across the repos than to build out a team to manage the custom tooling required for a gig plus sized monorepo

Plus, even google doesn’t have a true monorepo. Chrome and Android are not in the same repo as search for example. Find your seams and manage them appropriately