I'll try my hardest to add to your confusion. Taily is spot on. I don't think he spelled out that when the vapour bubbles form they slow down transfer of the heat (coolant transfers heat better than vapour).

As to whats better, viscous coupling or thermo fans? Generally on old tech go with the viscous coupling (BMW still use them). On old tech you can't engineer in the safety systems that kick in to protect your engine when thermo fans fail ie blown fuse, damaged wiring, worn parts etc. These failure modes are remote < than 1 in 1,000 but still. A viscous coupling gives plenty of warning before it fails and is more a slow degradation over time

Viscous couplings are noisier and less efficient (you won't notice the difference in efficiency its that small) but more robust. If robust is absolutely important then go for a fixed blade but they are much more inefficient.