Honestly, I really dont trust any digital thermostat controllers. They all have horrible failure rates and accuracy issues. I also do not trust heaters with them built in. Most the time it's just some crudely designed microchip board with budget Chinese cloned chips... Destined for failure. There is probably a more expensive option for larger tanks that is like a hardwired thermostat and element made to last. Or maybe not...
Funny, I don't trust current bimetal switches because of cheaply made contacts.. <a href="https://www.plantedtank.net/forums/images/smilie/icon_wink.gif" border="0" alt="" title="Wink" >:-)</a>
Just waiting to pit and fuse..
Mercury bulb type may be fine but can's see how you really incorporate it in a tank.
As far as I'm concerned.. all made in China.. <a href="https://www.plantedtank.net/forums/images/smilie/icon_sad.gif" border="0" alt="" title="Sad" >:-)</a>
There are plenty of high quality electronics or solid state probes and circuits.. just a matter of, like you know "good enough for the market".
Using something like a Honeywell process controller (solid state) and platinum based sensor (RTD) is ideal.. but $$$$$$
for those really really concerned.. shopping eek bay and researching "industry grade" controllers at pennies on $ is err.. "fun".
I've got one of these laying around somewhere.. <a href="https://www.plantedtank.net/forums/images/smilie/icon_smile.gif" border="0" alt="" title="Smile" >:-)</a>
Needs a new Pt sensor though.. Orig failed since it wasn't water proof.. Human error on my part..
Replacement sensor is $70 minimum..