You can't directly measure V because for a superconducting system, R is exactly 0, and I is nonzero. You can't easily have components in line with the current either for the same reason unless they're superconducting -- and practically making switches, diodes etc for superconductivity applications is a gigantic pain.
Quench protection systems exist to make the whole of the magnet dump its current uniformly, not in one tiny area, and are very complex. This is a neat extension of the idea with B field monitoring as a proxy for I.
Ok, I was under the impression that V is slightly nonzero during quenching, but I suppose the nonzero R parts get shunted by parts that are still superconductive.
The article mentions the initial phase of the quench is very slow for these kind of superconductors, and hence voltage or temperature sensors don't work well to detect the quench.
Seems to suggest the associated voltage is just too low to reliably measure (due to noise) until it's too late?
And what do you do when you have detected the beginning of a quench? Shut down the magnet? Isn't this impossible in many cases, eg a fusion reactor?