it means it may have vibration issues....the factory one is so heavy so it can absorb the vibrations and shocks of the engine running...ie, the shocks of the crankshaft and rods when they suddenly change direction 7000 times per minute....it helps to smooth the output of the engine. i have yet to se anyone that has had issues with this..but it is a risk you must be willing to take when you install the pulley.
IMO....the Perin pulley is the BEST one out there..not too light, and fits nicely. try not to get a underdriven set, if you ar gonna be driving the car on the street...the underdrive effectively slows you accessories down, reducing the output of any crankshaft driven accessory. IE..your AC compressor wont turn as fast, and the ALTERNATOR wont either. if you keep the revs up..you wont have that much of a problem...but a lot of guys i have seen do this install have had issues with charging systems afterwards.
again...just my "slightly" educated opinion..