First of all, it's not an open diff, so most of the time, the mechanical limited slip is doing most of the work. I'd be surprised if I4wd was kicking in much at all in the sand.
When it does kick in, you can feel it pulsating the brakes. It takes a second or 2 for it to start working. Maybe not a big deal to some, but it's screwed me a few times in technical rock crawling. I've also had it flat out not climb a rock. I've also not been impressed in the snow, because at times you are trying to not spin and dig, but the system relies on spin to work.
So to be fair, for most people, riding typical trails, it's fine. Once you get into more hardcore use, it falls short. It's also an infinitely more complex system than any locking diff. My system was actually not functioning from new and it took my dealer 2 weeks to fix.