I have bought 4 different ATO all used and am using 3 at the moment. I haven't had any problems with any of them. Two of mine has switches like this AutoTopoff.com
this is the safest one I have one in a sump and one slipped into a 24gallon jbj. My 55 I only have a gallon jug for top off and need to refill it once a day. One of the 24 gallons I have a gallon jug on and fill it about every third day. The other 24 I have a 2.5 gallon gallon tank on it and fill it once a week. This is similar to the other top off Auto Top-off System | GreenAquarium
the draw back to this one is getting it adjusted just perfect and keeping it that way because of the suction cups.
I have never had one over flow. I use the aqua lifter pump they aren't the strongest but seem to do pretty well for the price, I do find it works better on the sump other than the cubes since it doesn't have so far to pump the water.
If you are nervous, you can do like I did for the first few weeks only use a gallon jug so it doesn't overflow your tank. I don't use anything bigger because of space but know if I am going to be gone I can put 5 gallon bucket. They are very easy to set up like I said I bought all mine used and no instructions.