There has only ever been one consistently top tier pokemon in Mix and Mega, Primal Groudon, the undisputed king in all gamemodes he's allowed in. All other pokemon rise and fall in viability over time as the metagame adapts and changes with time.
Landorus-Therian is no exception to this rule. He too has risen and fallen from grace over the four years of Mix and Mega. I remember when Mix and Mega were in its infancy. Stumbling around blind we ranked Landorus-Therian S rank, and for what set? Mega Mewtwo X. It seems laughable now, but at the time there were calls for a quick ban. With time the metagame changed, and Landorus-Therian wasn't top tier again for a while. But Landorus-Therian is not the only pokemon who has experienced this! Cresselia was also suspected at the start of ORAS, and now it's not even ranked anymore.
This is a lesson. We can't, and should not, ban something unless we believe it's broken in this meta and all the metas to come. The fact that multiple pokemon that were unbanned at the start of ORAS isn't even top tier anymore should tell us to be humble, and how much the metagame can adapt, adjust and overcome challenges if given time. Though it shouldn't be ignored that some of these mons were banned again. Deoxys-D, Genesect, Lucario, Aegislash, Darkrai, Shaymin-Sky, Manaphy and Cresselia are all examples of pokemon we either assumed broken and banned or wanted to be suspected at one point who are no longer top tier(Should be noted how much the pidgeotite ban helped). A lot can change over time, not everything is Regigigas in disguise.
When a pokemon with 120/170/180 bulk, magic bounce, recovery and calm mind is not even ranked anymore that should tell us something about the power levels in Mix and Mega and the criteria for suspecting anything should be, right?
And that brings us to Landours-Therian. It's clear to me that Landorus-Therian has never been as strong as it is right now. It's never been more threatening and harder to build around. It's found its place in the metagame, moving away from meme-sets and closer towards the wallbreaker role it excels it. I don't know why it's not S ranked at the moment, but that's neither here nor there. Most of Landorus-Ts strong sides have already been excellently been explained by the many active contributors to Mix and Mega and of course
Chloe. It's honestly made me very uncertain on what my vote should be. On one hand, I strongly believe what I said above but on the other Landorus-Therian is impossible to build around with the explosion SD set. It absolutely destroys any kind of balance/stall build and centralising the metagame around offence. I'm just going to post some calcs I threw together real quick, Landorus-T w/ Adamant facing the most common defensive checks in the metagame - assuming SD on the switch. Keep in mind most of these are dead after rocks even if Lando is jolly.
SABLENITE +2 252+ Atk Aerilate Landorus-Therian Explosion vs. 252 HP / 252+ Def
Skarmory: 275-324 (82.3 - 97%) -- guaranteed 2HKO
LATIASITE +2 252+ Atk Aerilate Landorus-Therian Explosion vs. 252 HP / 252+ Def
Magearna: 338-398 (92.8 - 109.3%) -- 56.3% chance to OHKO
SPLASH PLATE +2 252+ Atk Aerilate Landorus-Therian Explosion vs. 252 HP / 252+ Def
Arceus-Water: 777-915 (175 - 206%) -- guaranteed OHKO
SHADOW SHIELD +2 252+ Atk Aerilate Landorus-Therian Explosion vs. 2522 HP / 252+ Def Shadow Shield
Lunala: 475-559 (45.4 - 53.4%) -- 40.2% chance to 2HKO
AGGRONITE +2 252+ Atk Aerilate Landorus-Therian Explosion vs. 252 HP / 252+ Def
Buzzwole: 552-649 (132 - 155.2%) -- guaranteed OHKO
SABLENITE +2 252+ Atk Aerilate Landorus-Therian Explosion vs. 240 HP / 216+ Def
Mandibuzz: 657-774 (156 - 183.8%) -- guaranteed OHKO
SABLENITE +2 252+ Atk Aerilate Landorus-Therian Explosion vs. 252 HP / 252+ Def
Shuckle: 399-471 (163.5 - 193%) -- guaranteed OHKO
MULTISCALE +2 252+ Atk Aerilate Landorus-Therian Explosion vs. 252 HP / 252+ Def Multiscale
Lugia: 366-432 (87.9 - 103.8%) -- 25% chance to OHKO
SABLENITE +2 252+ Atk Aerilate Landorus-Therian Explosion vs. 252 HP / 252+ Def
Toxapex: 523-616 (172 - 202.6%) -- guaranteed OHKO
SABLENITE +2 252+ Atk Aerilate Landorus-Therian Explosion vs. 252 HP / 252+ Def
Zapdos: 357-420 (92.9 - 109.3%) -- 56.3% chance to OHKO
SLOWBRONITE +2 252+ Atk Aerilate Landorus-Therian Explosion vs. 252 HP / 252+ Def
Blissey: 1017-1197 (142.4 - 167.6%) -- guaranteed OHKO
And just to get my point across;
SABLENITE +2 252+ Atk Aerilate Landorus-Therian Explosion vs. 252 HP / 252+ Def
Zygarde-C: 595-702 (93.5 - 110.3%) -- 62.5% chance to OHKO
The problem is clearly explosion, but since we're not doing complex bans then there's a pretty good chance, despite what I said earlier, that I will have to vote ban. However, I want to point out some of Landorus-Therians flaws.
First of all, and this is the biggest, it doesn't sweep very well. For all its power and might it's simply a tiny bit too slow in the metagame to consistently sweep the same way its competitors, Magearna, Lucario, Cobalion and Zygarde can pull off. I often found myself getting revenge killed by Arceus-W, Keldeo, Golisopod, Kartana, Terrak etc. Since I couldn't fit rock polish on the set it functioned more as a wallbreaker than a sweeper. The lack of speed means you will get revenge killed if you try to sleep early on, even with the immense bulk Landorus-T has on the physical side.
Second, it's not very unpredictable. Since, in my mind at least, the Salamencite set/Altarianite set is the only ones top tier Landourus-T has lost a bit of its versatility. Pdon is a better rocker, Arceus and Zapdos are better defoggers and you already grab momentum with explosion so U-turn isn't all that necessary.
Honestly, I started of certain that Landorus-Therian shouldn't be banned but now I'm not so sure. The absolute lack of defensive answers makes me very uneasy and not sure what to vote. I don't see how anything can "adapt" to SD explosion when it kills even resisted hits. There are good arguments on both sides, so for the moment, I'll reserve my vote until the time is up.
I look forward to hearing the discussion on this!
Good luck on the ladder guys, nice to see MnM is still active!