....
(If you want double insurance on 3-Phase you can hook a simple $20
electronic phase monitor up after the starter contactor, which will
detect a dropped phase, rotation reversal, or big voltage imbalance
and open the contactor even before the overload heaters do, in well
under a second. Cheap insurance.)
....

Here's something I hadn't heard of. Are they readily available from a place
like Grainger or McMasterCarr?

Would it be a good idea to wire one of these into the Estop circuit on large
machines? If this device drops, open a NC contact in front of MCR? (Master
Control Relay)

I had a weird one where my generated leg dropped out after my hydraulic pump
started on my huge CNC lathe. (pump comes in on Estop reset) The pump kept
going on two legs (as all three phase motors will) until I tried to bring in
my 20 hp spindle. It popped the main circuit breaker in the machine. No big
deal, but this means the control lost power also.

Karl