I prefer one which will shut off the power if it drops too low, or too high. And a timed restart delay when the power comes back on.
I've seen campground voltage drop below 105v several times. Especially in hot weather when all the units ACs kick on.
I prefer an external to an internal hard wired unit. Replacing an internal hard wired unit is more difficult and expensive if you have to do it.
I've had two surge protection devices self destruct (one in my home, one in my workplace) when lightning was hitting in the immediate area. That's what they are supposed to do - sacrifice themselves to save the more expensive electronics down stream.
My 30Amp one for my old trailer only tripped once - during a thunderstorm.
Maybe it was 'wasted money' - but two other rigs in the CG lost TVs, microwaves, AC in that surge.