Putting a mutex around a global variable doesn't change the fact that it is still a global variable. Memory access might be safe but you get spaghetti code and

Putting a mutex around a global variable doesn't change the fact that it is still a global variable. Memory access might be safe but you get spaghetti code and combinatorial state explosion due to all the potential side effects. Allowing singletons for edge cases is fine but with no proper way to enforce it except code review you really have now idea what the underlying code might potentially do.