I can tell you from my experience that they are a simple mechanical device and that every time on my 2 ton hilux with 31” tyres, they never missed a beat bringing all 4 tyres down to within 1 psi of each other.
When I changed vehicle they brought all 4 tyres to within 1 psi of each other but a much lower Tyre pressure. The only difference was the second vehicle was 3 ton 80 series with 33” tyres. Once I reset them they still worked at the desired pressure and within 1 psi of each other. You are welcome to make any thing you want of this information. However this is what I found happened. Feel free to discuss your own air but do not discount my experiences.
Yep, a simple mechanical valve that pushes against a spring. The valve has a certain sized surface (fixed) and the spring sets the tension against the valve. It takes a certain amount of pressure to push the valve open against the spring, depending on the spring tension.
So there are only a couple of things that vary the pressure required to open the valve.
The size of the surface of the valve plunger - nope it's fixed
The spring tension against the plunger - nope it's fixed, once you have set it correctly.
So yes, it's a very simple mechanical device, with very little to go wrong.
What can be a bit of a trap is not tightening them down on the valve stem properly. This causes the escaping pressure to be divided between the air escaping through the valve, and the air escaping around the valve. But it will cause the staun to close sooner, because it's only seeing a portion of the total pressure. But then the air is still escaping around the Staun, so you'll probably end up dropping the pressure too low anyway.
Also, if you deflate your tyres while they are very hot, once they cool down, the pressure in the tyre will be a lot lower. Conversely, If your tyres aren't overly hot, and you drop them to a certain pressure, once you drive them partially deflated, they'll heat up and the tyre pressure will increase.
I originally set mine up for a Toyota 4Runner, then about 5? years later changed to a Troopcarrier. The pressure were always with .5 to 1 psi. A few years ago I did reset them, because one of them seemed to be dropping the tyres to 2psi difference. But since then, they work perfectly.