It is common for macro shooters aficionados to mount their lenses in reverse, that is, to thread a camera-mount specific adapter onto the lens' filter thread. Lenses from about 50mm to 100mm are commonly mounted this way.
But while a lens' main body makes one solid and strong block with its "normal" (camera body) mount side, going toward the threaded end of the lens is a mechanically less strong focusing mechanism, either mechanically or electrically controlled. Also the thread depth is often shallow as its primary design is to accommodate a very light filter.
How strong is this threaded side of the lens, and how big a lens can be mounted in reverse? And as a corollary, can mounting a lens in reverse damage the focusing mechanism?