I suspect the chances of Nankang being utterly unable to make a road tyre that is anything other than terrible yet happening upon the ability to suddenly make a semi-decent track day tyre are somewhat remote, but I guess stranger things have happened.
I suspect they are getting popular because on internet car forums, 'cheap' will always subconsciously win through.
Thats how the Falken 452, the most over-rated tyre on the internet, became so popular. People were so desperate for them to be good, because they were half the price of everything else, that they managed to convince themselves and everyone else they were
If you've had decent results with the Federals why not stick with a tyre you know?