The alum does not fatigue any more than steel. Plus, they have more material according to the ones listed in the link you gave. Aluminum has come a long ways in recent years. My dad has worked most of his career in aluminum and has shown me some pretty neat things. Look at this way, do you have any type of aluminum wheel holding your car up? It does make a difference (big) if you have cast or forged. Also, most of the wilwood brake kits come with an aluminum hat to connect the rotor to the spindle or axle. There you are trusting the complete ability for your car to stop on two to four pieces of aluminum. I have a set for my car that I have not put on yet (cars in a million pieces yet), but they look really nice. Takes a few pounds off as well. Just my opinion though.
