My calcs and experiences are against MegaLuke with Adaptability and etc. Other than that I agree with what you're saying overall and I've never claimed that Volcarona doesn't have it's own problems (it's considerably worse on Pokebank OU because Heatran is popular). It's just that ChaosToTheMax and many others ask what are the best checks and counters to Lucario and some people are saying things that are quite misleading.
I have elaborated on how a Lucario check I have been using from 1900-2200 has been working well for me, and will not go into this any further as I have done my best to explain why it works. The October stats are just bonus evidence which demonstrate that when you take into account the spikes and the defogs and everything else that comes with the game, Volcarona ends up being one of the top10 things to scare Luke off or take an attack then kill it.
I took a look at the usage stats, and I'm not so sure they're completely reliable anymore. Volcarona above Dragonite as a check? Venusaur nowhere to be seen? What the hell is Chandelure even doing showing up in this section when it doesn't even see that much
regular play, much less as a check to Luke (it needs Scarf, which less than 50% do)?
Mega-Venusaur and bulky Gyarados are much better checks or outright counters to Mega-Luke than bulky Volcarona, and they're constant pains in my ass all the time. They resists Fighting moves, aren't weak to most anything else Luke carries, and most notably have
good physical bulk. Blaze Kick or Ice Punch? Nope, Thick Fat. The only way Mega-Luke is getting past Mega-Venusaur is with Zen Headbutt / Psychic for the respective sets (Thunderpunch / Stone Edge for Gyara), but it is really not worth using them as they expose themselves to more threats. Mega-Venusaur can then retaliate with Sleep Powder, Gyara can use Thunder Wave, and both threaten with Earthquake. Stopping both physical and special MegaLuke is a feat very few Pokemon boast.
Mega-Heracross can take +2 Close Combat (Mega-Hera needs to evolve though, and may not like even unboosted CC to the face unevolved), and can KO with Fighting STAB; Spiritomb can tank physical Luke's attacks and burn Luke or take advantage of Steel's Dark neutrality for a stronger Foul Play / unresisted Shadow Ball; Scarf Landorus-T is a pretty strong check to MegaLuke as +1 Extremespeed is the best thing Luke can go for before getting KOed by EQ. All can take heavy damage from Special Luke though.
Luke is easier to stop if you manage to scout its coverage moves. If Lucario lacks Bullet Punch, Aerodactyl and Blaziken can revenge it (without even going Mega), as can Mega-Gengar (needs to have evolved already). Lukes lacking ESpeed can be revenged by Talonflame and faster Steel-resists, like Greninja, Mega-Manectric and Jolteon. Lukes lacking Ice Punch are stopped by many bulky Flying-types (especially Gliscor and more defensive Landorus-T), and Lukes lacking Crunch can be stopped by Aegislash, Slowbro and woot Chandelure.
I know the special set has different responses, but it doesn't have as much coverage and power as the physical set. However, the less specially bulky switch-ins, such as Slowbro and Gliscor, must be more cautious against Luke as a whole.