Of course it's safe, provided all pilots exercise good airmanship, including, but limited to:
Good lookout,
Good situational awareness (in particular their position related to other traffic, instrument approaches, a/f circuits, controlled airspace, terrain etc),
Understanding their responsibilities, and the limitations of ATC provision outside CAS,
Understanding their own limitations (human factors) and the kind of problems they can cause.
Of course aircraft should be allowed to fly outside CAS, so if it's safe for a C152, why not a B757? You could argue that a B757 is easier to see, and has a more highly trained and experienced crew (see above).
Airproxes occur in CAS as well (shurely not?)!