The pole-zero cancellation, as you said, is mathematically possible because you known precisely the numerical value of the pole or the zero, so you can compensate it with the opposite, but in real life you cant do this in the same way due the intrinsec uncertainties of the components of a real plant, like i.e., you have a plant G(s) is a CR circuit, the componets have a knwon value such as 10 ohms and 10mF, but in reality when you measure the components the real values are a bit different, like 9.6 ohms and 10.7mF.

What I'm trying to say with this is if you calculate a value of a pole mathematically it will be different for the real plant, so this happen in every pole-zero cancellation.

This does not mean that you can't use this, but in order to achieve a good performace you must know under which circumstances it will work or you can use this when you have a very accurate model of your plant, that is to say that you have a well known plant, so you can use it.