Autotransformers are advantageous over normal transformers as they are cheaper. Autotransformers are used generally for voltage conversion of equipments from one voltage to another such as from 110V to 220V or vice versa.
However, the autotransformer does not provide isolation between the primary and the secondary. Hence, there may a need to connect external filtering or suppression circuits. Thus, in the event of a failure of the insulation between the turns of the winding, there are chances of the primary voltage appearing on the secondary.
Another aspect which needs to be checked is the neutral point. If the neutral point is not at ground potential in the primary, the secondary neutral wil also not be at ground potential.
The autotransformer has higher voltage stability and better overload transformers than the ordinary transformers.
A Variac is a variable autotransformer with movable taps. Thus, it provides variable output voltage for a steady input voltage. The taps can be adjusted by a movable knob.
Difference between Autotransformer, Variac and Dimmerstat