The same thing that would happen if my a/c in my house fails and just out of sheer luck the thermostat fails also. The temperatures will rise. The thing is I use 2x UTH & and a dual light system. from 6am-6pm the day bulb is on and from 6pm-6am the night bulb is on. With the night bulb full power it's 86 degrees, and with the day bulb 75% it's 88, will sometimes go to 90 on the Acurite but it's a Acurite issue, because the substrate stay's 88-89 from the day bulb. Now sure, if I just had one UTH then I could run a thermostat but for 4 different things it just wouldn't be feasible because then issues would occur. Also if the a/c went out at night it wouldn't make a difference because my house temps stay the same, now if it went out during the day it could be an issue, but luckily I work overnight so I'm home during the day so I honestly don't worry. Plus I monitor my temps via a wireless webcam that I can check from a browser or any phone so I can see the thermometer and I know if that thermometer ever read 90 then it's 92 under the day bulb. Sure it's not perfect but anything can go wrong at any point.
Thanks for chiming in Mark. I do keep my room a set temp, it may vary one degree either way but other then that it's stable. I'm hoping to find something that will act as a fail safe and cut off the power if it see's a certain temperature which would cut off my UTH and bulbs. I'm sure they have something I just haven't found it yet.