The problem with tyre pressure monitors that replace the dust cap is that you are depending on an airtight seal between the valve tube and the cap. To operate, they have to disable the actual valve.
So yes, they will tell you of a tyre is soft, but they are the most likely cause of the problem they alert you to!
My car detects soft tyres by counting how many times the wheels go round. If one starts turning more than the others (because its effective radius, and therefore its circumference, is smaller) it tells me. But I have to delve deeper into the stored values to find out which one is soft!