I don't think the rex gearbox is weak by anymeans, it's just that those who bought them assumed WRX = WRC car, and drove them accordingly, and ten years of full hektik driving has taken its toll as you would expect. If you manage to find one that's been cared for and unmolested, i doubt that you'd have a problem.
I'm going to go out on a limb here, and say that the majority of cars with gearbox issues all have uprated clutches - The clutch is designed as the weak link in the drivetrain, and when it gets broken, people tend to replace it with heavy duty, million plate replacements. This just shifts the weakness onwards, to the synchros or other parts. And when they go bang 'it must have a weak gearbox'. No, it's just being mistreated, and fettled inappropriately.