[demick]

I'm no expert, but as you would expect, the UDP slows down the speed of everything it drives: A/C compressor, power steering pump, water pump, and alternator.
Lots of testing has been done by many people to see if there are downsides to the UDP. I remember seeing several posts by Jake during his testing before he approved and started selling this pulley (search these forums and you will see this stuff). I don't recall seeing anyone having any negative effects. In fact, there can be benefits to running these items slower for those who track their cars. The power steering pump specifically has issues at higher rpm's that the UDP solves. But at the other end of the extreme, this probably isn't for the person who spend lots of time idling in stop and go traffic in 100 degree heat with A/C and headlights on.