Comment by leoedin

Thermostatic mixers make sense for continuous flow applications - showers, filling a large bathtub for example. But I don't think they make sense for a kitchen tap - where you might be filling a small basin.

The goal is that the water in the basin is the correct temperature, not the stream filling it - which means you need to compensate every time based on how much cold is already there before the hot water arrives. And that varies seasonally.

The time it takes for hot water to arrive tends to be a big proportion of the total time the tap is running, so you actually don't want controlled 40C water once hot water arrives. Perhaps if you had instant hot water, this would be useful.