Digital thermostats work on electrical resistance which is more accurate than bimetallic thermostats. Also bimetallic thermostats basically only switch at one temperature whereas most digital thermostats can handle a range. If you intend on using a kettle for both coffee and tea, that range is desired since the ideal temperatures between coffee and tea is quite significant.
Pop-disc bimetallic thermostats switch at only one temperature, but the property of bimetallic expansion is in no way limited to switching at a single cutoff point.
I'd be willing to make a large bet that a majority of people who brew both coffee and tea do so using boiling water or close to it. Black tea (breakfast tea) is best made with freshly boiled water, while I find coffee made with anything below 90C to be too weak - I generally use water made shortly off the boil when I'm using an Aeropress.
I have zero doubt digital thermostats are more accurate. That wasn't the point.
Thermocouple, RTD, Bi-metallic strip/coil, Silicon Bandgap, Thermistor, and I'm sure there are a few common types that have slipped my mind. Your statements are a gross over-simplification.
