Material properties of carbon compared with aluminium:
-Carbon is lighter (1.8 g/cm^3 for carbon, 2.7 g/cm^3 for aluminium)
-Carbon is stiffer (65-ish GPa for aluminium depending on grade, 65-110 GPa for woven carbon, up to 300 GPa for UD carbon)
-Carbon is stronger (275 MPa for aluminium, up to 1800 MPa for woven carbon, up to 3700 MPa for UD)
-Carbon does not fatigue like aluminium
-Aluminium is far more resistant to impacts and day-to-day knocks
But it really depends on the fork. I've had an aluminium fork on a modern road bike which was unbearably stiff, but yet the aluminium fork on a Vitus 979 soaks up almost all road chatter. Though it's quite flexible, it is probably the most comfortable fork I've ever ridden. I currently have an early 2000s Trek carbon fork which has no compliance in it at all, but I used to have an Alpina fork which was very smooth to ride.
Simple fact is most carbon in the bicycle industry is badly designed with material in all the wrong places, so it's almost always over engineered. This results in a product that is far stiffer than it has to be, which results in an uncomfortable ride. But, likewise, a badly designed aluminium fork is no better and there's plenty of those out there too.
