Generally, the advantage of casting is that it melts aluminum alloy at high temperature and pours into a mold to form as one so the designing flexibility on the disc is pretty high thus it’s easy to mass produce and cost effective. On the other hand, from technical stand point, it’s hard to reduce weight since it’s difficult to form into thin metal. Also what was formed through casting is often uneven and particles are not cohesive therefore its strength is not good enough.
On the other side, forged wheels are made by running a lump of aluminum or alloy into high-pressure pressing machine thus aluminum particles spread and formed according to wheel shape and it is stronger (Fiber Flow). Because of the way they are produced, forged wheels can be made light-weight. It seems like it’s perfect but there is a definite weakness of “It’s not flexible from design perspective. It has already been explained, forging process uses high rate of weight to form the shape consequently, it is not suitable for complicated cosmetic face design. Forging also has a disadvantage of being more expensive due to multi-processing.