The problem with traditional derailers and cassettes is the inability to shift at a stop. Not a big deal for recreational applications, a very big deal for utility applications, especially in an urban environment. This is the same advantage that any internal hub has (which are older and more traditional than derailer drive trains).

As for slushy, well... I was skeptical too, but the reports are that it is anything but. It operates very differently from traditional CVTs, which use friction belts and such. This looks like it should be a lot more efficient, and that's what people seem to be saying.