DoN. Nichols writes:

A rectified autotransformer is simply a DC power supply that is
ground- or neutral-referenced. Nothing wrong with that. Indeed, it
protects against certain faults. And it would be reckless to permit a
floating supply to operate in the presence of faults.

Not if the hot and neutral are interchanged -- anywhere between
the breaker box and the Variac.

It is still ground referenced in that event.

Consider the full-wave rectified/capacitor-filtered output of an
autotransformer, with the wiper set to deliver a DC potential of V
across the + and - output terminals.

If the AC hot and neutral are wired properly, the + and - DC output
terminals are at +V/2 and -V/2 with respect to the neutral.

If the AC hot and neutral are reversed, the + and - terminals are each
at line voltage but offset by +V/2 and -V/2 respectively.

Neither case presents a problem to an isolated load.

I suppose if you thought of this as a way to get, say, a battery charger
for your car, where the load is not isolated but connected to the
vehicle chassis, then yes, it presents a big hazard. Or if you think
that a low (DC) voltage output is safe to handle bare-handed.

This is strictly for fully isolated loads.