Important to note that, as I'm sure you know, materials react differently to milling or cutting than they do at high speed impacts.

In almost every instance, the harder something is, the more brittle it is. If you flung a diamond hard enough, it would either chip or break entirely.

Obviously if it's too soft, it'd bend or shear off on impact.

I'm not in the "no planes" club as there have been a lot of first hand accounts that there were, but the way those towers (and a certain other building) went down was definitely suspect, especially considering it was built to withstand such impacts to begin with.