Well, different hardness and different alloys for different purposes. At the velocities we shoot, a softer alloy with more lead content works good. Fit and lube are also very important. If a bullet is too hard and driven at slow speeds, it may not bump up, seal the bore and could lead the barrel. Especially if it is on the small side for the bore.
A too soft bullet that is driven too fast can also lead the bore. I have shot pure lead bullets at nearly 2000 fps in rifles with no gas checks. Can be done with paper patches.
I have also had extremely hard bullets that leaded terribly due to poor lube and poor fit.
When I was casting bullets for the cowboy crowd, I strove for a Brinnell hardness of 11-12. No one ever had a problem with leading that ever told me.