I'm not saying they are always wrong -- though in time that will happen -- I'm saying they are never RELIABLY correct, and they can be WAY off, with little or no warning.

I used to be a dealer for pH / ORP controllers for large commercial pools, and am very familiar with the 50 year old sensor technology involved. The electronics just adds a layer on top of the sensor output; they cannot compensate for the intrinsic limitations of the sensors themselves!