Fundamentally the weak interaction is harder to calculate (even with a computer), b/c the coupling constant for QED is so small it allows many orders of perturbation series to reliably be calculated, before asymptotic renormalon behaviour starts corrupting your data. The Fermi coupling being larger doesn't give you that reliability.
Of course there are other issues in the calculational difficulty, some of which are based on the accuracy of experiment, and some technicalities in the weak sector that are quite challenging. First of all the propagator is massive, which adds extra stuff to the calculation, second of all the process is now distinctively axial in nature. Essentially this makes drawing feynmann diagrams for V-A interactions an enormous chore, best left for grad students =)
(note I am pulling an all niter grading papers, so forgive the brevity and conceptual mistakes I am likely making)
(incidentally I hate the weak interaction. I find it remarkably unbeautiful compared to QED or QCD. I also hated attending conferences with the electro weak experamentalists, as invariably some bayesian statistics debate would rear its ugly head and everything would degenerate)
I never meant to imply anything else by my post.
