The media have said the same about nearly every major sporting event in history. Then it usually all goes smoothly and the media end up with egg on their face. Over and over again this happens. Classic examples are the London 2012 Olympics and the Glasgow Commonwealth Games.
There's no doubt that this event will test Cardiff, but as long as the trains run smoothly, I can't foresee any major problems. In my admittedly biased opinion, I believe it's laid out to be one of the best cities for sporting events. No long travels to the stadium from the city centre (which really does kill the mood) and quick access to bars, the bay and the parks for the celebrations/drowning of sorrows.
I for one can't wait!