First, it's become popular to say that the WRX has a weak tranny. Whether or not that's true depends upon what you mean by weak.
In the vast majority of cases where the tranny goes kaput, driver abuse is to blame. People like to abuse a car like the WRX. It's fun.
Probably a lot of people are wasting first gear in particular trying to dump the clutch at 5k rpm. Secondarily I'd guess that people are slamming too hard into 2nd gear, but since second gear is pretty tall, it probably will survive.
Also remember that most people never have problems with their cars, and as such they don't post "My WRX ran fine again" to every BB they can find. In other words, take what you hear from the internet and from friends with a grain of salt.