'course, the viscous unit can fail "free" and as such not provide good
cooling, but that's only one failure mode - if it fails "stiff" then you get
the fan running more than you want, which is less than optimally efficient
but doesn't risk blowing the engine.

My observation of viscous units is that the most common thing to happen is
that it doesn't free off when it should, although they can no doubt fail to
lock up again when they should as well.

There's also more of a roadside bodge factor available - if you find that
the viscous unit is not locking up and the engine's overheating, then to get
you running you just have to find a way of jamming it so it's solid, and
have a permanent fan.  A blown electric fan motor can't be made to go
again...