Yes I've read about the isolation ones and lots of conflicting articles about it.
Yes, Confusing info. In Australia where I was and I suppose in other countries with 240 V outlets, the electrical courses included the variac use.
In old days, extension leads were often hand wired, so the neutral prong was not really standard.
Using a variac to get from 240V to, say 120V or 24 VAC can raise a false sense of safety because the uninformed can probe on 24 VAC thinking it is safe.
But the "0V" terminal might be at 240 V wrt ground earth, with the other terminal at 216V wrt ground.
Often the variac has input to tap to allow setting higher than input voltage.
I have one like that here in USA, it allows up to 140 V out with 120 V in. Also the socket is old type, unpolarized, no connection diagram.
Using a GFCB (RCD) or isolation transformer upstream of variac is a good plan. I have dedicated GFCB for all outlets on electronic bench and another for the outside workshop.