I don't think most of the cracked ones can be attributed to impacts. Remember, it also holds the steering rack. If you look at the cracked one in the link on post #13, that's pretty much how the ones that I've seen crack, but they will also crack where the height adjuster goes. And don't forget they also bend and will throw the front toe off. But hey, if it bends enough you won't need rack spacers to fix the bump steer.
It's been decades since I measured the difference, but it was something like 2 lbs difference between the factory steel one and the cast aluminum one. The steel one is surprisingly light. At the absolute lowest part of the car body.
To each their own.
Look at Tyson's post #12 here. He's pretty much seen the same thing that I have.
https://forums.pelicanparts.com/porsche-911-technical-forum/340596-alloy-vs-steel-crossmember.html