Keldeo can viably run HP flying
Name a soecific one?
Azumarill takes 50% from Earthquake, gyarados also doesn't need to mega evolve.
Scizor kills Talonflame w/ banded Knock off after rocks:
I think you're right about that last one, but it's not always that easy to counter anything in OU
We rely on checks for the most part
Keldeo vs Venusaur
252 SpA Choice Specs Keldeo Hidden Power Flying vs. 232 HP / 4 SpD Mega Venusaur: 168-198 (46.7 - 55.1%) -- 69.5% chance to 2HKO
252 SpA Keldeo Hidden Power Flying vs. 232 HP / 4 SpD Mega Venusaur: 112-132 (31.1 - 36.7%) -- 67.6% chance to 3HKO
252+ SpA Mega Venusaur Giga Drain vs. 0 HP / 0 SpD Keldeo: 284-336 (88.1 - 104.3%) -- 25% chance to OHKO
Keldeo needs to hit Venusaur on the switch with HP Flying to get the 2HKO, and needs some kind of boost to do so (with hazards), ergo be at +1 on the CM set or holding Specs. The former is tricky to get to while Venusaur is still around, and the latter is exploitable locked into what is otherwise a specific coverage option. HP Flying also means Keldeo gave up his best tool vs Celebi. even Keldeo's best option against Venusaur gives him a shaky chance 1v1 and makes him exploitable in context of a team.
252 SpA Choice Specs Keldeo Icy Wind vs. 252 HP / 220+ SpD Celebi: 134-158 (33.1 - 39.1%) -- 99.8% chance to 3HKO
Azumarill vs Gyarados
-1 252+ Atk Choice Band Huge Power Azumarill Play Rough vs. 88 HP / 4 Def Gyarados: 216-255 (61.1 - 72.2%) -- guaranteed 2HKO after Leftovers recovery
-1 252+ Atk Choice Band Huge Power Azumarill Play Rough vs. 0 HP / 4 Def Gyarados: 216-255 (65.2 - 77%) -- guaranteed 2HKO
252+ Atk Choice Band Huge Power Azumarill Play Rough vs. 0 HP / 4 Def Mega Gyarados: 494-584 (149.2 - 176.4%) -- guaranteed OHKO
252+ Atk Mold Breaker Mega Gyarados Earthquake vs. 172 HP / 0 Def Azumarill: 164-194 (42.7 - 50.5%) -- 2.3% chance to 2HKO
Gyarados has to Mega for that chance, and without the boost he's depending on an incredibly unlikely damage roll. CB Azumarill meanwhile will 2HKO virtually every offensive Gyarados set with Play Rough, while BD can check Gyarados by setting up alongside it and having a good chance to survive +1 EQ with Sitrus Recovery.
+1 252+ Atk Mold Breaker Mega Gyarados Earthquake vs. 92 HP / 0 Def Azumarill: 246-290 (67.5 - 79.6%) -- guaranteed 2HKO
+6 252+ Atk Huge Power Azumarill Play Rough vs. 0 HP / 4 Def Mega Gyarados: 1322-1556 (399.3 - 470%) -- guaranteed OHKO
Talonflame vs Scizor
Scizor needs the Rocks to OHKO Talonflame with anything on the band set, while Talonflame outspeeds and OHKO's with Flare Blitz. On the other hand, without the band (to change to Bullet Punch for a 1-2), Scizor needs high rolls to get the 2HKO.
And the thing is, Aegislash doesn't have counters, which you could say of Zard-Y and Landorus-I. But Aegislash has very few consistent checks as well: He has a couple checks to different sets, but almost nothing that can be thrown on a team and say "I have Aegislash covered" as opposed to "I have SD and Crumbler Aegislash, but not SubToxic covered". Aegislash requires you to scout his set to play around him right, and sometimes you can find you weren't adequately prepared for the set you're facing because you had to patch a weakness to Lopunny, Landorus, or even another Aegislash set. This inability to adequately prepare for Aegislash's variety of sets without using several teamslots is a constraint on teambuilding, arguably similar to the problem we have with the huge number of threats. Aegislash's sets all share his benefit of really good defensive typing and stats, but the roles they can be made to play mean SD Aegislash and SubToxic Aegislash are about as distinct as Boosting Landorus-T and Gliscor are.