anything can fail
your screws could fail and dump your MH into your tank and kill it too
my ATO setup cost me $40 and I got one with dual switches and a relay so that it can handle bigger pumps (even though I only use an MJ1200)
this is the one I have
http://www.aquahub.com/store/product26.html
I think one reason people have failure is by:
1. not using backup switches
2. not using a relay to handle the larger pumps
3. using gravity feed w/ solenoids that salt builds up on
4. not using snail guards on their float switches
it consists of 2 float switches (one to turn it off and one is an "emergency" shut off one, so at least it can never fill so much it over flows), wiring, etc, MJ1200, various plumbing (I use rigid for everything), and the wrapping paper reservoir (best thing ever...holds about 9 gallons), snail guards (mandatory!!!)
also if you're that worried about disaster you could just make sure to have a small amount of RO in your reservoir...that way if the setup fails (which it wont) then you will only put a little bit extra in and won't crash (this solution is obviously not viable for vacations)
but so far mine has been running for over a year without ever having a single problem