I do not think it has been proven in a 5000lb 4x4. It is also new, unlike the 5sp that has been around for years.
By no means am I saying the ZF is a bad box, I have a ZF 6sp in my car and it is the best auto gearbox I have ever owned. I am however more cautious when it comes to new boxes and the problems they can have, as they are all comuter controlled and sometimes too smart for their own good.
I am just hoping the 8sp is out when I am ready to buy a SRT8, as I would like that