Hi work,
The low voltage I'm comfortable with is not below 108. The unit cuts out at 104 volts. I.E. it is useless for me personally.
In 12 years representing 1200 days of camping, I've never seen voltage higher than 123. Safe zone is 108 to 132 volts. (i.e. plus or minus 10% of 120 volts).
The unit won't protect from a direct lightening strike.
Some autoformers come with surge capabilities. That might be useful to me.
I'd bet that for every person who has a surge device that there are 10 who don't. Yet I see very few comments about damage to 30 amp RV's.
50 amp is a different kettle of fish. They need one--and most folks DO NOT have one. Again there are few reports of damage.