Specialization, what's unsound about it?

I've used specialization recently in one of my projects. This was around the time I was really getting I to rust, and I actually didn't know what specialization was - I discovered it through my need (want) of a nicer interface for my traits.

I was writing some custom serialization, and for example, wanted to have different behavior for Vec<T> and Vec<T: A>. Found specialization feature, worked great, moved on.

I understand that the feature is considered unsound, and that there is a safer version of the feature which is sound. I never fully understood why it is unsound though. I'm hoping someone might be able to explain, and give an opinion on if the RFC will be merged anytime soon. I think specialization is honestly an extremely good feature, and rust would be better with it included (soundly) in stable.