Kash To be fair, it IS a single point of failure. However, the general reliability of the method (drip tray plus drain) and device (liquid or pressure sensor) negates having to do more for very little improvement.
To be honest, I've only ever come across one machine (in 40 years) that had two detectors. That was because the machine was large enough to have two discrete cooling "sectors" which did not exchange either the secondary coolant (glycol) or the primary internal coolant (air). Each drained to its own detector.
Converting energy to motion for more than half a century