Not a geezer, so I never really played Ubers when it was around (the inception of GSC, period). But Lugia doesn't get Reflect, Ttar isn't a worldbeater but does have some potential if you keep it basic, Steelix definitely has potential if people don't run many Fire Blasting/Hidden Powering variants of things, and Mew needs a couple of variants to exist if it's going to be the world-beater it's advertised as. Also, mixed attacking is silly, since everything has mammoth defenses and speed, and everything gets goddamn recover.
I feel like GSC Ubers would only be good for the novelty of "hey, I'm playing GSC Ubers." Outside of that, the meta seems really stupid, imo.