My big problem with Keldeo lies in the fact that he's dependent on Scald and/or Prediction to do his job well with the sets I've played (though I admit to having little experience using or fighting the SubCM set). Keldeo's coverage movepool is pathetically small, compensated for by his high Special Attack... on Super Effective targets. If Keldeo clicks the wrong move, he's hitting a target with a non-STAB Base 60 move in some situation. Dependence on HP for coverage causes an odd situation for Keldeo, since teammates can deal with most of the HP targets (Celebi, Gyarados, Venusaur), but even as defensive as they could be Keldeo hates giving them a free turn (Sub/NastyPass, DD/Sub, Leech Seed/Synthesis respectively) to potentially play around the switch-in. Keldeo's STABs hit hard on neutral targets, but Keldeo NEEDS to make sure he's hitting the target neutrally or super-effective to avoid guzzling momentum on his Choice sets.
As for the SubCM sets, my issue is that Keldeo either can't boost fast enough to beat every Special Attacker, since many outspeed and/or carry Psyshock to hit him super-effectively on his weaker defensive stat, wearing him out before he can boost high enough. I understand SubCM as more a late gamer sweeper, but some of the mons that could take advantage of the SubCM set seems like quite a bit to remove. To name some that come to mind (Keldeo is at 87% at best after Sub and two turns of Leftovers):
(I'll assume these come in on the turn Keldeo Subs, or that they switched in on his first boost, meaning he might get a second one)
+2 252 SpA Keldeo Scald vs. 0 HP / 0 SpD Mega Gardevoir: 201-237 (72.5 - 85.5%) -- guaranteed 2HKO (Burn is 12.5% per turn, so w/out hazards Keldeo would never KO)
232 SpA Pixilate Mega Gardevoir Hyper Voice vs. +1 0 HP / 4 SpD Keldeo: 330-390 (102.1 - 120.7%) -- guaranteed OHKO (Subbed)
232 SpA Mega Gardevoir Psyshock vs. 0 HP / 0 Def Keldeo: 338-398 (104.6 - 123.2%) -- guaranteed OHKO (Two CM boosts)
+2 252 SpA Keldeo Scald vs. 4 HP / 0 SpD Mega Altaria: 125-147 (42.8 - 50.3%) -- 1.6% chance to 2HKO
252+ SpA Pixilate Mega Altaria Hyper Voice vs. +2 0 HP / 4 SpD Keldeo: 204-242 (63.1 - 74.9%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Pixilate Mega Altaria Hyper Voice vs. +1 0 HP / 4 SpD Keldeo: 270-320 (83.5 - 99%) -- guaranteed 2HKO after Leftovers recovery
192+ Atk Pixilate burned Mega Altaria Return vs. 0 HP / 0 Def Keldeo: 220-261 (68.1 - 80.8%) -- guaranteed 2HKO after Leftovers recovery (Can put a dent even if Keldeo gets the Scald burn on its switch)
252+ SpA Mega Venusaur Giga Drain vs. +1 0 HP / 4 SpD Keldeo: 188-224 (58.2 - 69.3%) -- guaranteed 2HKO after Leftovers recovery (Venusaur will heal off the damage from Keldeo's attacks at +1 while breaking Subs)
Now, this is just me thinking of things off the top of my head that beat Keldeo once he's boosted factoring in potential burn, but I don't consider that dependable enough to boost the sets a full Sub-Rank, especially in a gap from A+ to S. Scald only has a 30% chance to burn, and regardless of our bad experiences with the RNGods, that's not a statistically reliable number. Tyranitar isn't considered a counter to Gengar because Focus Blast has a 30% chance to miss, the same way I don't consider Azumarill a less reliable check because Scald has a 30% chance to burn in the two uses to switch in. The odds are a bit shakier considering Keldeo only wants one burn vs T-tar wanting two misses, but regardless, the odds of getting the Scald burn are 51% (0.3 + 0.7*0.3 = 0.3 + 0.21). Keldeo is gambling on a 50-50 to win if it stays in, and the equivalent of a Focus Miss if it retreats on a failure.
Even then, Keldeo still loses to the burned Azu example, meaning he had to leave the opponent can Double Switch
252+ Atk Choice Band Huge Power burned Azumarill Play Rough vs. 0 HP / 0 Def Keldeo: 292-345 (90.4 - 106.8%) -- 43.8% chance to OHKO
Scald is an annoyance to the opponent and a nice fringe benefit to Keldeo, but if that's what's pushing him to S-Rank, I'm really not sold. Counters and Checks are picked without considering Hax, so Keldeo being highly ranked because it has hax that can beat them seems an unfair way to judge his viability. I know having checks and counters are not the only grounds for dropping, but Keldeo has issues with what I'd argue are some of the most important defensive mons in the game (we're discussing Altaria for S right now), being stopped by those as a Wallbreaker or needing them broken down as a Lategame sweeper is a serious issue for him.
A+ rank is still respectable, and Keldeo fits in well enough there for what he does do well. His STABs do achieve decent neutral coverage, and on those neutral targets he hits like a truck from the Special side, which I find are much harder wallbreakers to prepare for. I probably got something wrong on SubCM, so don't hesitate to correct me on that if I misunderstood its use. I just feel like people are overselling Scald a little for Keldeo's viability. He's good, but not the same monster in gen 5 when he was pulling numbers like this. Keldeo's just not something I find myself preparing specifically for, whether it or cores including it, as I do Lando-I or Mega Metagross, or even Zard-X or Altaria in A+.
(252 SpA Choice Specs Keldeo Hydro Pump vs. 232 HP / 0 SpD Rotom-W in Rain: 207-243 (69.2 - 81.2%) -- guaranteed 2HKO after Leftovers recovery)
As for the SubCM sets, my issue is that Keldeo either can't boost fast enough to beat every Special Attacker, since many outspeed and/or carry Psyshock to hit him super-effectively on his weaker defensive stat, wearing him out before he can boost high enough. I understand SubCM as more a late gamer sweeper, but some of the mons that could take advantage of the SubCM set seems like quite a bit to remove. To name some that come to mind (Keldeo is at 87% at best after Sub and two turns of Leftovers):
(I'll assume these come in on the turn Keldeo Subs, or that they switched in on his first boost, meaning he might get a second one)
+2 252 SpA Keldeo Scald vs. 0 HP / 0 SpD Mega Gardevoir: 201-237 (72.5 - 85.5%) -- guaranteed 2HKO (Burn is 12.5% per turn, so w/out hazards Keldeo would never KO)
232 SpA Pixilate Mega Gardevoir Hyper Voice vs. +1 0 HP / 4 SpD Keldeo: 330-390 (102.1 - 120.7%) -- guaranteed OHKO (Subbed)
232 SpA Mega Gardevoir Psyshock vs. 0 HP / 0 Def Keldeo: 338-398 (104.6 - 123.2%) -- guaranteed OHKO (Two CM boosts)
+2 252 SpA Keldeo Scald vs. 4 HP / 0 SpD Mega Altaria: 125-147 (42.8 - 50.3%) -- 1.6% chance to 2HKO
252+ SpA Pixilate Mega Altaria Hyper Voice vs. +2 0 HP / 4 SpD Keldeo: 204-242 (63.1 - 74.9%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Pixilate Mega Altaria Hyper Voice vs. +1 0 HP / 4 SpD Keldeo: 270-320 (83.5 - 99%) -- guaranteed 2HKO after Leftovers recovery
192+ Atk Pixilate burned Mega Altaria Return vs. 0 HP / 0 Def Keldeo: 220-261 (68.1 - 80.8%) -- guaranteed 2HKO after Leftovers recovery (Can put a dent even if Keldeo gets the Scald burn on its switch)
252+ SpA Mega Venusaur Giga Drain vs. +1 0 HP / 4 SpD Keldeo: 188-224 (58.2 - 69.3%) -- guaranteed 2HKO after Leftovers recovery (Venusaur will heal off the damage from Keldeo's attacks at +1 while breaking Subs)
Now, this is just me thinking of things off the top of my head that beat Keldeo once he's boosted factoring in potential burn, but I don't consider that dependable enough to boost the sets a full Sub-Rank, especially in a gap from A+ to S. Scald only has a 30% chance to burn, and regardless of our bad experiences with the RNGods, that's not a statistically reliable number. Tyranitar isn't considered a counter to Gengar because Focus Blast has a 30% chance to miss, the same way I don't consider Azumarill a less reliable check because Scald has a 30% chance to burn in the two uses to switch in. The odds are a bit shakier considering Keldeo only wants one burn vs T-tar wanting two misses, but regardless, the odds of getting the Scald burn are 51% (0.3 + 0.7*0.3 = 0.3 + 0.21). Keldeo is gambling on a 50-50 to win if it stays in, and the equivalent of a Focus Miss if it retreats on a failure.
Even then, Keldeo still loses to the burned Azu example, meaning he had to leave the opponent can Double Switch
252+ Atk Choice Band Huge Power burned Azumarill Play Rough vs. 0 HP / 0 Def Keldeo: 292-345 (90.4 - 106.8%) -- 43.8% chance to OHKO
Scald is an annoyance to the opponent and a nice fringe benefit to Keldeo, but if that's what's pushing him to S-Rank, I'm really not sold. Counters and Checks are picked without considering Hax, so Keldeo being highly ranked because it has hax that can beat them seems an unfair way to judge his viability. I know having checks and counters are not the only grounds for dropping, but Keldeo has issues with what I'd argue are some of the most important defensive mons in the game (we're discussing Altaria for S right now), being stopped by those as a Wallbreaker or needing them broken down as a Lategame sweeper is a serious issue for him.
A+ rank is still respectable, and Keldeo fits in well enough there for what he does do well. His STABs do achieve decent neutral coverage, and on those neutral targets he hits like a truck from the Special side, which I find are much harder wallbreakers to prepare for. I probably got something wrong on SubCM, so don't hesitate to correct me on that if I misunderstood its use. I just feel like people are overselling Scald a little for Keldeo's viability. He's good, but not the same monster in gen 5 when he was pulling numbers like this. Keldeo's just not something I find myself preparing specifically for, whether it or cores including it, as I do Lando-I or Mega Metagross, or even Zard-X or Altaria in A+.
(252 SpA Choice Specs Keldeo Hydro Pump vs. 232 HP / 0 SpD Rotom-W in Rain: 207-243 (69.2 - 81.2%) -- guaranteed 2HKO after Leftovers recovery)