Pure lead balls do perform on game well, however if there is heavy bone, a hardened ball resists excessive expansion and for that reason, some fellows hunting moose and elk prefer a hardened ball.
In countries where dangerous game is high on the list of desirability, hardened balls were the requisite projectile. In order to be accurate, elongated balls needed to be of pure lead, to 'take' the rifling. If pure, they failed to penetrate as well as a hardened spherical ball. For this reason, elongated balls (bullets) did not gain popularity until self-contained projectiles along with breech loading mechanisms for large and powerful rifles were developed. Thus, in a self-contained ctg. loaded at the breech, the hardened elongated projectile could be larger than the groove diameter and thus engage the rifling, producing the required accuracy.
I don't find WW alloys to be any more difficult to cast than pure lead. If indeed, it is not casting well, the addition of a small amount of tin will help.