This is becoming a beat-up topic. Somebody should have added this to an FAQ...
1. HOAT is ethilen glycol based coolant. HOAT = Hybrid Organic Acid Technology and refers to the additives.
2. According to DC and their FSM, it is safe to mix a HOAT coolant with the old green dyed one. It is not recommended though due to the reason of loosing HOAT benefits (long-life).
3. DEX-COOL is OAT, not HOAT and is used primarily by GM. There have been horror stories around about dex-cool, however many people suspect that those are mostly related to cooling system neglect. There are millions of GM vehicles on the road, and they are not falling appart due to the dex-cool. By the way, DC insists on keeping your overflow coolant bottle full. I suspect, this is to make sure there is no air in the system.
4. AFAIK, HOAT has been used for a long time now in Mercedes vehicles. How many horror stories has anybody heard about Mercedes coolant failure?