All 4x4 systems cannot be all things to all people, so there are significant trade offs. Lots of lift, ride quality, etc. The factory system is built with the ESP/traction control in mind, so it will provide much better control at highway speeds when in AWD mode.
Mercedes has themselves in a bit of a pickle. They have been working with Oberaigner (OB) for decades, and they incorporated their design into the sprinter. The 4WD sprinter has never been crazy popular, so they were happy to let OB make the parts and let MB install them. Flash forward to recent years and the 4x4 is much more popular. OB can't or won't make the investment to increase their parts production. MB has lots of money invested in certifying the OB sourced 4x4 sprinters. I would bet the OB actually partly owns the parts designs, so MB can't just ask another supplier to make them. So now MB can choose to stick with OB at limited quantities, or they can switch suppliers, change designs, and pay the $$$$$$ to certify a new design in half a dozen major markets.
Toss in a recent pending design change (and the cert required) and its easy to see why there is so much delay with MB.