If you go through Benson Leung's G+ page, most of the things will be clarified. The Problem is that since this is all new technology, actual impact is still unknown. While QC3,0 or 2,0 is not directly compliant with USB type C, there is no problem when a QC charger is in use to charge a USB Type C device.
Lets see the scenario - when one end (phone connection) is type C and other end is USB type A (normal) then the bundled charger should work just fine as it draws higher power and USB type C can sustain the higher flow. (Agreed that it still is no compliant)
Now the same cable if used to connect any other USB device, typicalls a PC or another old non QC charger, there might be a problem. As this legacy device may not draw power at higher flow but the phone with usb type C might demand it, hence it will be left to the cable to negotiate the correct flow of current between two ports. If the cable is a bad one (so many of them on Benso Leung's page) it will fail to negotiate a proper charge and this might damage the legacy device with USB type A. (It will be forced to draw more current than it is supposed to)
So safer option is to use bundled cable with bundled QC charger for charging purposes only. However use a certified cable for connecting to PC or any other legacy chargers or devices. I am using AUKEY USB type C to type A cable which is also certified by Benson Leung. Its available on Amazon.de as well as Amazon.com. Xiaomi's cable is not tested yet and hence cannot be sure if its a good one. I saw one report of cable with Mi4S being tested and it turned out to be a bad one so not sure what is bundled with Mi5.
Please do not bash my confusing explnation and what I am saying might be wrong as I am not an engineer and I do not understand watt and current etc in detail. I have just derived from Benson's explnation what is important to general user.
Note - I used bundled cable with legacy charger and apparently the charger is damaged (does not charge any more) This could well be a coincidence.
