Unmonitored simply means if there's a failure of some component of the ILS (LOC, GS, DME if installed, etc...) there WON'T be an alarm in the ATC facility.
So, theoretically, if you perform an ILS approach in IMC, and, let's say, the LOC goes off line, nobody will be able to confirm it was the transmitter that failed (as opposed to perhaps your receiver, or other equipment).
Would I accept an unmonitored ILS in IMC conditions? The conservative answer is, no. As any resulting incident/accident could be considered "careless and reckless".
But honestly, in good vis, with high ceilings, daytime conditions, I can argue the risk is mitigated. Judgment call. That's why we get the Big Bucks!
