So yes, an Adamant +2 Mega Medicham will always OHKO an offensive Gyarados after Stealth Rock, which is some of the simplest and easiest support out there. ghost_dog97 wasn't extremely specific about the example, he just said that it has the power to OHKO a Gyarados after being Baton Passed a Swords Dance. It turns out that Mega Medicham can do just that with pretty minimal support, so he was right. But honestly, can we stop arguing about petty things like this? Seriously, it's like every time ghost_dog97 posts something, there's this collective knee-jerk reaction to try to argue with random stuff he says. Even when he technically isn't wrong, someone always tries to find some way to spin the argument so that he is.
Oh, it has nothing to do with him. It's just a bias of mine; I don't consider "after being Baton Passed a boost" to be a very compelling argument for a Pokemon, given that it's something that literally every Pokemon can receive. It could be Baton Passed to Mega Aggron, Mega Abomasnow, whatever. A discussion of a Pokemon's viability shouldn't be contingent upon Baton Pass; rather, it's more of a boon than anything. No one doubts that when Scolipede passes an SD and a Speed Boost to Mega Medicham, things are going to be hurting, but at the same time, the same is probably going to be true for 90% of physical sweepers receiving the Baton Pass. I mean heck, other fighting types love being passed SD.
+2 252 Atk Life Orb Terrakion Close Combat vs. 4 HP / 0 Def Gyarados: 308-363 (92.7 - 109.3%) -- guaranteed OHKO after Stealth Rock
+2 252 Atk Adaptability Mega Lucario Close Combat vs. 4 HP / 0 Def Gyarados: 345-406 (103.9 - 122.2%) -- guaranteed OHKO
+2 252 Atk Life Orb Infernape Close Combat vs. 4 HP / 0 Def Gyarados: 265-312 (79.8 - 93.9%) -- guaranteed OHKO after Stealth Rock
Edit: Yes I am aware that most of these, most notably Terrakion, would have moves they'd rather use against a Gyarados but this is just for the sake of argument.
Pretty much any physical Pokemon likes getting Baton Passed a Swords Dance. I'm just of the opinion that whether or not a Pokemon should be viable should be discussed without Baton Pass support, because it's generally a given that once a Baton Pass is received, things are going to be hurting. That'd be a separate discussion about the viability of Baton Pass itself, and certain Pokemon acting as passers.
And when we throw in the context, it ultimately came down to a discussion of Drain Punch vs. Hi Jump Kick on Mega Medicham. And the underlying point was that Drain Punch does require a bit more support than HJK in order to secure some crucial 1HKO's. Especially since HJK's frighteningly high power is part of Mega Medicham's niche over its competitors; the overwhelming power it possesses is what makes Mega Medicham as good as it is. Forgoing that, despite not having as much risk (what with HJK's nasty side effect), does produce a noticeable decrease in damage output.
I honestly don't see why Adamant Mega Medicham is so taboo, to be honest. Yes, the extra speed can be important from time to time, but it's not like it's mandatory. Genesect runs Scarf the vast majority of the time, in which case you'll lose. Excadrill tends to be Adamant at least a small majority of the time (the spreads in the usage stats add up to ~52%, and there's no telling how many other slower spreads aren't represented). Fast Gliscor hardly even exists. Close to all Lucario run Lucarionite which makes them too fast anyway, and regular SD Lucario dropped in popularity when Mega Gengar was banned and teams like MikeDecIsHere's Gas Pedal were no longer usable. Plus, it's generally Adamant. Jolly/Naive Landorus-T with max Spe is pretty rare outside of Choice Scarf sets. You might beat some sets of stuff like Hydreigon, Haxorus, Kyurem, and Kyurem-B, but their usage percentages range from "uncommon" to "what happened to you?" It's useful for speed tying base 100s like Volcarona, Charizard, and Salamence, but a lot of those guys commonly run Spe boosting moves or Scarf sets. Bottom line is that Jolly is far from mandatory when so many of the Pokemon between that 88-100 base Spe area probably outspeed you anyway or common run slower sets that you can outspeed with an Adamant nature. Jolly is probably better since there are still times where that extra speed will come in handy (better safe than sorry), but the raw power of an Adamant Mega Medicham cannot be ignored.
Taboo it certainly isn't, but it's certainly something for which results will definitely vary. The Adamant vs. Jolly and Modest vs. Timid argument has been around forever, and I won't deny that it's largely a personal preference thing. That said, the number of Pokemon that can check you is affected by that 10% difference in speed. Call it paranoia on my part; getting checked by a Lilligant on one of those rare occasions when you don't use a Speed boosting nature is something that comes back to haunt you, and definitely makes a person stick to Speed boosters when given the choice.
My 'Nam Flashbacks aside, I find it comes down to the Pokemon. When Lando-T is acting as an offensive pivot rather than a sweeper or revenge killer, yes, it prefers a bit more bulk over speed. From what I've seen most Lucario tend to be running Timid/Jolly nowadays, though I'd be lying if I said I knew the exact margin. Excadrill, I'll admit, surprised me to see how popular Adamant is lately, but eh. That said, I'd agree with what you're saying; Adamant does have the highest power, but I've already been one to air on the side of caution, and go for Jolly to be safer. I suppose if you're going for an all-out wallbreaker, Adamant certainly has its merits, but at the same time... Jolly's definitely something that can narrow the number of checks if even only just.
Plus, at the end of the day, I reference my signature
Not the Weepinbell fusion part though. It comes down to personal biases, and I'm one to be strongly biased in favor of speed boosting natures.
