Hi Riccardo,
Honestly, I don't see a concern here. What is broken can be fixed or reverted. It is not "clean" but still we are referring to master branch which is equivalent to development. It is not release neither it is a "stable" branch.
But this is exactly the state in which I don't want to see the master branch. Namely, having non-reviewed code pushed to it. Sure, you can fix or revert the changes, but often times the commits lack a proper description. One advantage of a PR is that you are encouraged to describe your changes and explain why they are relevant.
Best, Hugo
