That was a Volvo problem and had nothing to do with Viscous coupling.  
Viscous coupling is an old tried and true technology that was not
implemented adequately.  There is nothing to guarantee that the Haldex
system will become unreliable in the future.  It is computer based and
its integrity depends upon the computer and its sensors.  We all have
experienced computer or sensor problems.  They are expensive to repair.  
I know, I have replaced input speed sensors, throttle position sensors,
and output speed sensors on an 850.

I have a 2001 Volvo XC70 with the old viscous coupling system that has
served me very well.  Viscous coupling is very sensitive to tire
diameter mismatches.  The tires need to be frequently rotated.  If used
properly it will serve very well.

On an Subaru I had with viscous coupling, it would actually pulsate the
power if the tire mismatch from front to rear was too great.  It wanted
the best (largest) tires in front.  If the front tires wore too much, it
thought that they were slipping and would transfer power to the rear
tires.  At an intermediate stage it would pulsate.  The solution was to
rotate the tires.  It did no damage to the AWD system, just was annoying.