Main problem with Porygon-Z is that Scizor easily switches on any move (except HP Fire) and resist his main STAB - Tri Attack and does shitload damage with Bullet Punch or Pursuit on switch. Tyranitar also isn't good news and he only fears HP Fighting. Metagross is another good switch. In steel heavy metagame Porygon-Z doesn't have an easy time in being usefull. To be honest I'm more surprised Porygon-Z still being OU and not Porygon2 in his place, as Porygon2 has some good niche in OU unlike Porygon-Z, as some pokemon use Choice Scarf better then him like Latias, Gengar or Starmie. Specs maybe isn't outclassed, but still with all those steels it's not much better and Nasty Plotter is just too slow.