They are pretty close to bulletproof, and they have a backup float just in case, if I remember right.
IMO the biggest things to keep in mind are:
Tunze will keep your water level the most constant, because the optical sensor does not have to let the water drop "X" amount before coming on... a float switch has a certain amount of "travel" from open to closed, and the Tunze does not.
JBJ is a good unit, BUT there is one scenario in which it will be trouble (and I know because it happened to me). If you have a pretty large sump, AND you use a "slow" pump like an Aqualifter (which I do because I run the ATO through a Kalk reactor), it may not be able to keep the sump full.
The reason is, it takes a few gallons at least to raise a float switch on a large-ish sump (where the return area has a pretty large capacity). A JBJ will "time out" after anywhere from a couple of minutes to fourteen minutes MAX. This is a safeguard against flooding. The time allowed is internally adjustable. In my case, with my pump, fourteen minutes' worth of water would not bring the level all the way up and the ATO would time out on me. It will not "reset" until the float switch rises and shuts off the pump. I would come home and find the sump low, the pump off and the JBJ showing that the sensor was low.
I ended up changing it out for a Reef Fanatic (with no timeout feature), and putting the JBJ on my wife's tank (which has a small sump). No issues at all since.
FWIW, I am buying a Tunze next. I like the consistency, and the alarms that it has for things like high sump water level.