While I think solid state components are more reliable typically than their mechanical counterparts, please keep in mind a few things. (I did circuit board/surface mount repairs exclusively for almost 10 years.)
I haven't had any of those units apart, or checked their output, but there's only a few ways they can do what they have to do.
They don't simply reduce the voltage.
They switch the voltage on and off very fast, either via switching transistor, schottky diode, pwm controller, or some other means.
I have replaced all those parts before, lots of times. They have all failed, and while, I think they would have outlasted their mechanical counterparts if they had the same duties to perform, they simply don't. They are typically forced to do much more severe duty. It comes down to design, all parts can fail.
I use double and quad rancos because I just don't want a simple mechanical failure to cause the death of several of my pets.
I don't think they are more or less prone to failure, they just don't have any redundancy, so if they fail, they fail.
I personally also think that using a herpstat pro with no redundancy is also a bad idea. I wouldn't even use it to control all the devices in one tank, as I said, there HAS to be a component in there somewhere that can fail that will take all the outputs down, no matter how reliable it is. There's just no redundancy.

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