... a stable tuning fork

I forgot, they haven't formed a coalition; perhaps it is truly effective.

Baldwin
“Baldwin recommends the use of Dampp-Chaser systems
as a valid aid to ensure the longevity and
stability of our pianos”.

Bechstein
In environments characterized by wide fluctuations
in humidity, Dampp-Chaser humidity control
systems are of great help in preserving the
extraordinary tonal quality and value of
Bechstein pianos.”

Blüthner
“The Life Saver system certainly helps to control
humidity levels around the piano, to prevent
any damage and to preserve its value.”
Christian Blüthner-Haessler

Bösendorfer
“The adoption of Dampp-Chaser humidity control
systems allows for effective compensation of
climatic oscillations in the environment housing
the piano, and has a positive impact on the
stability and reliability of the instrument over time.”

Kawai
“Dampp-Chaser humidity control products
are the best and most well-designed systems
we have ever seen.”

Pearl River Piano
“The Pearl River Piano USA Group believes that a
Dampp-Chaser system, installed and maintained properly,
can improve the performance and longevity of
our pianos.”

I know that these systems are sold in America where there are areas in which a very low relative humidity level is reached. I don't think they refer to areas corresponding to temperate climates.

These are systems for conditions where those who buy and place the instrument at home must necessarily make compromises or actually experience significant humidity fluctuations; one thing is certain, to evaluate the usefulness of such a system, ten years of experience would be needed, just as with meteorological studies that never take into account time intervals of less than 30 years, so a 5-year warranty and everything else is welcome, but when possible, it is sufficient to increase humidity in summer and limit it in winter (then it also depends on the value and use made of the piano; if I were a teacher, perhaps I would think differently!!)