I use a lot of composite actions in the same repo as well as composite actions across repos.
Eg. `workflow1` and `workflow2` both call `composite/action`:
.github/workflows/workflow1.yml
.github/workflows/workflow2.yml
.github/actions/composite/action.yml
The only missing bit in that is a bit more support in a composite action: `if` and a few other keywords. Also, it's a bit annoying to access private across across repos.