What should I consider if equipment deliberately projects through the open rear?

Quite a lot more than whether the chassis physically passes through the opening.

You need to consider equipment support, total load, stability, centre of mass, rear connectors, cable routing, airflow and how much of the equipment remains outside the enclosure.

A lightweight switch extending 100mm through the rear is one thing. Now take the same principle reductio ad absurdum: put a heavy server into a shallow 12U 300mm cabinet and let it project 600mm out of the back while the cabinet is sitting freestanding on a worktop.

The server has now become an excellent lever.

Move enough mass far enough behind the cabinet and the combined centre of mass can pass beyond the cabinet’s supporting footprint. The cabinet can tip backwards until the projecting server itself meets the worktop or floor and rather abruptly brings the experiment to a conclusion.

That’s an extreme example, but the mechanics are entirely real. Significant rear projection changes the behaviour of the complete installation, so deep or heavy equipment needs proper support and the cabinet must remain stable under the actual load arrangement.