I have an old thermostatic valve that was attached to a used Calumet sink I
bought. I set it to 72F when I installed it, a year and a half ago, and I
check it every couple of weeks, so far I haven't had to adjust it. I'd say
it's within +/- 1F after a 5 min warm up. Perfect for B&W. I also have a
Hass electronic mixing valve for my color processor, it seems to be spot on
as long as the water is running. My incoming water is softened and filtered.

I have checked my valve against both a dial and a mercury thermometer.  Of
course, if you've ever checked dial thermometers in a school group darkroom
for example, no two in a group of twenty in the same tub of water will ever
agree.  But both the mercury column and dial thermometer seem to indicate
that once stable, the thermostatic valve is accurate and can hold the
temperature + / - 1 degree which is as much accuracy as I can determine from
the markings on the thermometers.

I added the valve when I built my darkroom three years ago.  I have treated
(softened) heavily mineralized water, and so far have experienced no
problems.

These valves come in "low flow" and "high flow" models.  For a home setting
with one or two taps, you definitely want a low flow model.  If a high flow
model is employed with small volumes of water, it will not be able to
accuractely maintain the temperature in a low flow setting.  High flow
valves are used in multi-tap settings like school "gang" darkrooms.

With any of these type of valve, you must allow the system to become and
remain "stable."  This means you run the water until the hot water pipe is
hot all the way to the valve, and the room temperature water has been
flushed from the cold line.  Then (and only then) will the valve be able to
maintain the temperature you select.  Temperature adjustment must be done
slowly so that you don't overshoot - avoid big adjustments and give it time
to "catch up" between adjustments.

Once the valve is stable, it will maintain the temperature very well as long
as there is sufficient flow.  If, for example, I just trickle water through
a print washer, it will begin to wander off the exact temperature.  However,
if I maintain a flow of 1/2 - 1 gallon per minute, it will do fine.

If you think this kind of valve is suspect, you can always get a manual
mixing valve with a built-in "well" or chamber for a dial thermometer stem,
and do it yourself.  But keep an eye on it!  You will have to adjust the
manual valve during your session. Trust me, this valve has made my darkroom
life much simpler.

I too have seen a couple of thermostatic valves malfunction badly.  One, at
a large well known photography program.  In that case, however, it was the
fact that the school's dishwasher used up all the hot water.  Of course if
there is no hot water, no amount of adjustment will cause the output water
temperature to rise beyond the termperature of the cold water supply, and
since someone had left the valve in the very "hot" position, when the water
heater finally supplied hot water, the valve happily passed it on.  In this
case, it was a water supply and operator error, not a mechanical one.  The
valve was, in fact, functioning perfectly althought the result was a
disaster.

The original post I answered was in regard to using a demand heater in a
darkroom.  Because the hot water output temperature of a demand heater can
vary quite a bit, I'd suggest using a thermostatic valve with one.  I use
mine with a nearby 50 gallon tank electric hot water heater (NOT a demand
heater) with excellent results so far.