Let’s first of all look at what constitutes a good disc. It needs to be flat, it needs to be strong, it needs to be able to stand a great deal of heat, up to nearly 400 degrees centigrade without warping, it needs to be very resistant to wear, it needs to be compatible with many different types of brake pads and it shouldn’t rust. On top of that, it needs to be as light as is practical too! That’s an awful lot of requirements and it is the reason that so many of them are so poor.
It is very easy to make a poor disc but very difficult to make a good one!