GHA isn’t a product thought for GH private organisations, you will find that every much needed feature for this use is very low in GH roadmap.
However I'd say that Github's been pretty receptive to feedback and has actively fixed almost every wall that we've run into (if we haven't been able to fix it for ourselves)
It doesn’t matter how smart you are with reusable workflows you will never get to a truly DRY setup that scales for dozens of repositories.
Another major pain is that we still haven’t private actions. It was due end of 2021 (maybe it is out now but I checked a couple of days ago).
Setting up runners to look after a pool of repositories needs elevated permissions.
GH offers a way to enforce a list of enabled actions but this does not work with private binary registries hosting pre built Docker actions. The only thing that could prevent you to pull software at runtime from the internet, which means, if you want to have a decent security posture all you are left with is referencing actions using the full git sha version.
Many common use case require hacks, which is fun for a weekend project but isn’t great for a large scale operation. An example is simply running a workflow dynamically targeting the folders containing changes. At the moment you have to create a job, generate a build matrix on the fly and pass it in input as the matrix to the actual job.
I'm looking forward to this landing too. In the meantime, though, checking out the repository that contains the Actions and referencing a local path works fine so this hasn't really been a blocker for us.
Edit: per sibling comment, it seems that this feature became available in the last few days. Nice!