Here are some calcs (these both assume neutral damage):
252+ SpA Choice Specs Adaptability Porygon-Z Thunderbolt vs. 4 HP / 252 SpD Blissey: 214-252 (32.8 - 38.6%) -- 4.4% chance to 3HKO after Leftovers recovery
252+ SpA Mega Charizard Y Fire Blast vs. 4 HP / 252 SpD Blissey in Sun: 220-261 (33.7 - 40%) -- 32.5% chance to 3HKO after Leftovers recovery
So Zard-Y has more turn 1 power than Specs Porygon-Z with its most powerful move. Banning Porygon-Z because of its raw power would be stupid. An advantage Porygon-Z has over Zard-Y in this meta is the ability to boost with Nasty Plot, but that doesn't change the fact that it can still be countered by Blissey with STAB Shadow Ball. This was just for comparison. The post I really wanted to respond to was this:
All those typings you mentioned are destroyed by landorus.
The fact that Porygon-Z and Landrous may not share a single common counter does not make either broken. Name one Pokemon in OU which counters both Mega Charizard-Y and Keldeo. Just because they don't share a counter does not make them broken. In type control, you can use Ghost / Normal Blissey to counter Pory-Z and Ghost / Fairy Chansey to counter Landorus. Hell, you don't even have to give a shit about Keldeo because both are immune to Sacred Sword. I can't even think of one special attacker which can break through both Chansey and Blissey in type control, and in OU you have to usually run at least three separate counters to deal with Zard-Y, Keldeo, and Landorus, so stall has a pretty good deal here in comparison. Combine those two with Avalugg which walls the vast majority of physical threats, and you have most of the meta countered with three teamslots to spare. You can use these three teamslots to patch up vulnerabilities to setup sweepers, Taunt / Substitute, and anything else which gives your stall team problems. I think the main problem with Adrian Marin's team is that it is too passive. Why the hell does he need Cress on his team when he already has Chansey + Blissey + Avalugg? He could replace that with something which can actually do some damage to deal with Pokemon to give stall problems. He could probably also replace Mew with something that is more immediately threatening. In OU or in any meta, you can't just rely on Toxic and Stealth Rock to do damage because other players will use Taunt, Substitute, Magic Guard, Poison Heal, etc. so you have to prepare for that when building a stall team. Stall doesn't need any extra help from banning Porygon-Z, Landorus, or Gliscor because wallbreakers and stallbreakers are part of any meta and stall teams should be expected to find a way around them.
I don't really think Porygon-Z is broken against offensive teams either. It has mediorce speed, no priority, and its bulk is not very good. It needs Agility to be effective against offensive teams, but then it isn't quite as stupid powerful and it is still weak to Sucker Punch. Finding the setup turn can also be difficult for it.
Landorus fits in the same box as Porygon-Z with a lot of these traits, so I don't see it as broken either. I think we need to play many more battles before we ban anything unless it is totally obvious that a Pokemon is causing the meta to completely revolve around it.