There is nothing wrong with plastic. Just because a toy has less metal part, it doesn't mean it's a bad toy. I bet you barely handle with injection plastic kit.
Yamato valkrie is more like an assembled plastic kit with lots of details like a plastic kit has. It's not a typical toy. They can have metal hinge on the shoulder, what you get in return is paint chipping. When I think about diecast metal, I can't avoid to think paint chipping off, discoloration, uneven seam. These 3 factors can easily translate to plastic as an solution.
Or
They can care less about articulation and design, to put in more traditional metal part, then you will have discoloration and chipped off paint.
There is no ultimate rules of thumb to judge a toy is a good toy when it has more diecast metal, it's a matter of balance between design and durability.