I would say a good 90% are a result of a lack of proper and complete documentation by the manufacturers. In the case of Asus RT-N56 the documentation is incredibly poor. I personally rely on the manuals that come with a device. From what I have read here, many issues could have been resolved...
Ok so it's just the documentation that sucks. I downloaded the manual as part of my research into whether or not to buy this router. No mention of having a DDNS besign Asus DDNS.
Also, I note that there is no VPN support according other reviewers and there is no mention in the manual either...