I prefer the option of separates.
I can rig the isolation transformer to how 'isolated' I need it to be, be it ground/earthed or not, and/or neutral/earth ground link variations
and be able to use the isolation transformer first, variac second in the chain, and vice versa
Put lamps, multimeters, clamps, scopes wherever required in the chain to indicate voltage in/out, current, faults etc
Separates are easier to check and service etc
The All In One is a nice no brainer (and looks cool
), but not for some total voltage control types like me