I think that the viscous fan is always rotating, so will produce a steady airflow over the engine.
Where an electric fan will wait until the engine starts to get too hot, then suddenly kick in and cool everything down before shutting off again.
Having said that, the viscous fan will stop rotating when you turn off a hot engine, causing heat soak, where an electric fan may continue running.
Pretty much every car on the planet has an electric fan these days.
I would guess that a viscous fan would in some cases be better in a vehicle used for extreme off road and deep water crossing where an electric fan may stall or get water ingress.