Antimony was originally used in wheel weights to make them harder. Tin was the original metal preferred for this application, but with rising prices, they switched over to Antimony. Originally everyone use to cast, and lead was very common, however due to restrictions becoming tighter, it was harder to find pure lead, so casters starting switching over to WWs. With this switch, Antimony became a common metal in their alloys.
The problem with antimony (SB) , is that it doesn't add anything beneficial to a lead bullet. Any alloy with SB can be made much better with the use of Tin. As people became more and more use to SB in there alloy, they just starting accepting the problems that came with it. Alloy with SB in it is almost always way to hard, for a lead bullet to properly seal in a barrel/chamber, it needs to be soft. Hardball alloy (with SB in it) does not allow proper bullet obturation.
People still use alloy with SB in it, but SB will cause your bullets to be way to hard, and very brittle. If you ever tried to cast a HP with an SB alloy, you'll notice when you recover it, all the petals will be completely sheared off, due to the brittleness problem.
If you cast, and actually care about a proper alloy, pure lead, with tin, is the way to go. You can harden it up to proper BHN with only tin, and it'll work just fine in about anything you put it in, as long as you have the bullet properly sized to bore, and use the proper lube and/or gas check.