I won't swear to be an expert on this but based on past posts the viscous functions as follows:
1. ECU or some such senses coolant temp below viscous heater operational threshold (see my note above) which
2. triggers relay R36 in Power Distribution Center which in turn engages the "clutch" on the viscous heater which
3. causes two or more friction discs inside the viscous to "rub" together which
4. transfers friction generated heat to the fluid (silicone IIRC) inside the viscous heater which
5. transfers heat the the engine coolant flowing thru the viscous heater housing to
6. the cabin heater core and then on to the engine block.
7. when ECU or some such senses coolant heat above the viscous operation threshold relay R36 is triggered to release the "clutch" on the viscous heater and it quits "working".
Important note - there is no "valve" on the cabin heater core so engine coolant is constantly circulating past the viscous, cabin heater core, and back to the block. Cabin heat level is controlled by a "blend" door. The point being that if your cabin heat is not working properly AND you have a good tstat then both cabin heater core hoses should be essentially the same temperature. A colder return line suggests a partially or fully clogged cabin heater core. Equal line temp but poor heater performance suggests malfunction of the blend door system not to be confused with the separate system that activates the doors that direct air flow to the various vent combinations.
As long as the all the above events that need to "happen" for the viscous to work it will work. As far as I know the silicone fluid cannot break down but whether it's possible to leak out I don't know although if it did I suspect there would be issues with the friction discs and then the viscous bearing. As noted the viscous does not produce much heat on it's own and stops producing heat well before engine operating temp. In a really cold situation with a really bad tstat I would not expect cabin heat to be "hot" rather it would be "warmish".
FYI the only viscous failure I've seen on the forum is discussed at
viewtopic.php?f=5&t=60567