I did forget mega venusaur, but it still takes a decent chunk from adamant HJK and it definitely isn't a safe switch-in unless it's already mega evolved. This is obviously a personal preference, but I consider double-priority revenge killing more important than the ability to KO one notable wall unless your team happens to be particularly weak to mega venusaur. Zen Heatbutt is more situational in that respect.
In terms of comparing Megacham to other famous priority users, there are a few reasons why I consider megacham better at the job than Scizor:
-Medicham doesn't get choice locked. This factor shouldn't be downplayed since being locked into bullet punch can be VERY bad, even if you end up revenge killing something. It often gives your opponent a free turn afterwards, and not having to worry about this is a huge plus.
-Bullet punch is resisted by more pokemon than the FO/BP combo. Unless you're up against Aegislash or Mega Lucario, the FO/BP combo has better neutral coverage.
-Medicham still has a ridiculous HJK and much more speed. It boasts much more power than Scizor could ever wish for.
-The "third turn" you mentioned shouldn't even be a factor. If Scizor and Megacham can't OHKO (or 2HKO) something with their priority moves, they will have to take a hit regardless. So why not just use HJK or Ice Punch? If the opponent switches in another pokemon on Bullet Punch, it's probably a pokemon that resists the move and doesn't care about getting hit with it twice.
Now I'm not going to argue that Megacham is a better priority user than Talonflame, but Brave Bird is one of the only reasons why Talonflame is even OU. It doesn't have a whole lot else going for it.
I highly suggest trying the set out before saying that these moves simply aren't good. Fake Out on its own isn't good, nor is Bullet Punch. Put them together, however, and you will see why this set is a completely viable option over coverage moves. Medicham is simply powerful enough to make non-stab priority work, unlike most other pokemon in the game.
First of all, and just to clarify I'm not hostile to your argument and I think you made some really good points, I do agree about the utility of BP. I found abandoning an elemental punch for BP was giving up too much coverage. That was until I was left with only unevolved Mcham against a +3+2+1 DD Weakness Policy TTar with 40-45% left, a NP togekiss at 40% and a full health Greninja, with rocks up on my side of the field. I thought I had lost until Mcham came in, straight up megaevolved, and proceeded to BP to Death TTar, Togekiss, and OHKO Greninja with HJK while eating up rocks, a couple of turns of sand and a scald. That was hilarious.
Now, down to the Fake Out and Adamant vs Jolly business. For the latter, I also generally rely on priority on the megaevolve turn (maybe to finish off a weakened mon), or, otherwise, I come in on an obvious switch, so that's not really an issue to me at all. However, I do think that the nature issue is way more important
after mega evolving. Not outspeeding base 100 (and something below) 100% of the time is kinda big, especially with your set (Ice Punch/FO/BP/HJK). With that set, Zard X can come in and DD with impunity on you or revenge kill you (and if you're switching out as he tries to revenge kill, something else is gonna take a good hit, even if resisted; if he starts sweeping, you'll have problems even when you'll come back using FO+BP, as he resists BP, but Zard X is a total monster anyway), Salamence (not as common as in the past, I'll give you that) can revenge kill, and so does Intimidate Arcanine (witch resist BP) even without using Espeed, as does +speed unevolved Luke etc. Kyurem-B and Haxorus will lose a good chunk of healt if they dare to come in, so that's not that relevant, if you have something else than can reliably finish them off (and everyone who chooses Mcham should have those kind of Pokemon). I run Jolly so I can threaten these kind of mons. Otherwise, they will outspeed you and straight up kill you 100% of the time or set up in case you've revealed your set entirely (and that's going to happen, because you're going to use FO+BP frequently, and fighting STAB is obvious).
Movepool-wise, I find, Zen headbutt very good. First of all, it's powerful, way more powerful than the punches. And it's damn useful when you can't risk HJK and an elemental punch would do nothing at all to the target (aka neutral damage to a fighting resist/immunity). Talonflame takes a lot if he dares come in on ZH too. Ice Punch has a lot of uses though, and I'm considering replacing ZH for IP. For reference, I run HJK/BP/ZH/Fire Punch, and I do believe that Fire Punch is the most useful of the elemental punches. I find FP more useful than Ice Punch, because of Ferrothorn (you'll otherwise have to risk HJK fearing protect) and Aegislash, especially if ShieldSword is lurking, menacing to come in and proceed to have you miss HJK and straight up kill you with Shadow Sneak. Fire Punch destroys Aegislash, and Aegislash becomes a big problem with your set. To add salt to the injury, that thing is everywhere.
If anyone is wondering, yes I've tried FO+BP, and I found the lack of coverage rather lackluster, especially on a supposed wallbreaker-to-be. I see you use more as a kind of a priority revenge killer, but the FO+BP combo has a lot of problems with ghosts and steels not weak to HJK. Just out of curiosity, as I'm interested in that kind of set, who do you pair him with?