OK, I'm sure this is overkill, and anyone who understands these things will think I am being ... well afraid. But here goes anyway.
1. Variacs have a serious safety issue of not being overload protected on the output side.
2. If/when variacs fail, it can be a very thermal event. Like burn your wood deck and siding kinda event.
Number 1 means no short protection, no ground fault protection, no zap pop whoops gotta flip the breaker. In outside, possibly damp conditions this means stuff should probably be a little off the ground and away from things that could drip on it, catch fire, etc.
Number 2 means you should have a way to get it well away from your expensive house/deck safely without having to reach over it to unplug it.
Overloading equipment voltage can lead to failure of any electronics in the path, as well as possible failures and even shorts on heaters and motors that aren't rated for it. Setting it when the line is low, then having an oven inside the house kick off and send the voltage up 10% (to 132) might not be something you are comfortable with on a hand-held heatgun, for example.
Only youcan keep the fire dept from helping with your hobby.