So I'm currently in support of banning Landorus-i from OU, but with a few stated caveats and stipulations as to why.
Firstly, I think you can only really define the brokenness of a pokemon within a particular tier based on the metagame it's currently in. I really resent people recycling Gen 5 arguments about why they chose to ban Landorus, since ORAS is an entirely different metagame and should not be held to the standards of any previous metagame. This also comes into play when people say "we should ban Landorus-i because it's a potent offensive threat and ORAS has too many"; this line of logic relies on some mysterious premise that having a lot of offensive threats is suddenly unhealthy for competitive play, or that because there are relatively more threats in ORAS than DPP or BW OU, we should be concerned.
Essentially, what I'm saying is that Landorus should be judged based on: whether it promotes a spirit of fair and equal play between players, the extent to which its presence centralizes the OU tier, and the effect Landorus has on the metagame relative to its capabilities.
I think it is generally agreed upon by both sides that Landorus promotes a spirit of fair play; there is nothing surrounding its use that affects competition between both players (e.g. Swagger, endless battles, etc). Whether Landorus makes the game equal for both players, however, is a bit blurry. Surely both sides are free to use Landorus, and given that it is fair, a player does not have to play around an opponent's Landorus in any extraordinary or unreasonable way. Scouting something's movepool and making double switches is nothing new or unique. Where Landorus may be deemed unequal, however, is in the extreme utility a player gains by running Landorus as opposed to a player who does not. Given that it has the tools and capabilities to be tailor made for success against any playstyle (less-so against offense, but nonetheless), there is a significant opportunity cost when opting not to use Landorus, and the momentum of a battle shifts significantly in the Landorus user's favor upon successfully sending it into battle. This inequality stems from the immense power and spammability of Landorus' main STAB - LO SF Earth Power, and the additional boost for all of its coverage moves which makes the performance of these double-switches and scouting measures incredibly more difficult than one would consider reasonable.
Taking all this into account, when we observe the current ORAS OU metagame, we generally see a trend towards bulkier balance and bulky offensive teams, most of which are heavily pressured by Landorus. This is crucial because, for instance, the earliest stages of XY OU allowed many other fast, powerful offensive threats like Deo-S, Greninja, Mega Lucario, etc. that made Landorus far less useful and relevant. Additionally, Landorus-T has since been "replaced" by Tank Garchomp as the bulky birdspam-check and SR setter of choice for many players. In this way, usage stats can be more useful, because while it is rather obvious that human irrationality and inexperienced players can skew which pokemon are used more and deemed "better" than others, Landorus has a unique problem in that it cannot be used alongside Landorus-T. To me, it seems like the decline in Landorus-T coincided very closely with the increased concerns about Landorus' effect on the current ORAS OU metagame. That, in conjunction with the shift towards generally "fatter" team archetypes makes Landorus all the more threatening - the metagame has shifted to one that heavily favors the wallbreaking power Landorus provides. Knock Off's buff is also extremely relevant to this suspect, as without Knock Off, Landorus is back to relying on U-turn, a sub-par option in my opinion, to beat Lati@s, Gengar and Chansey which otherwise check it well.
I'm going through this explanation so painstakingly slowly because I really dislike some of the arguments that other pro-ban users are trying to pass off as legitimate. Just saying "look at all the moves it can run!!!" isn't just a bad argument - it's an incomplete one. Tornadus-Therian is honestly one of many examples of a fast, reasonably strong offensive LO sweeper that can run a handful of coverage moves. However, no one is going on about how we need to ban Tornadus-T. Additionally, do not just say "Landorus has 115 Special attack, 125 Attack, is really stronk" because we can increasingly see that stats alone are a poor indicator of how much damage something deals. Greninja only had 103 SpA and never even ran a +SpA nature, yet it hit extremely hard.
What pro-ban should be stressing instead is the uniqueness of Landorus' tools as an offensive sweeper + wallbreaker: that Life Orb + Sheer Force + Spammable STAB/coverage deals far too much damage for most walls on bulkier teams, which constitute a large majority of common teams in ORAS, forcing these teams to run one of three viable checks in Mega Latias, AV Tornadus-T and Cresselia. Stop debating whether or not Cress or Mega Latias are good or bad; it is a horrible point that persuades no one and is subjective to you alone. Play on the ladder and actually illustrate to anti-ban users why Landorus' absence improves teambuilding options for these balance teams. Taking it on faith that we're better off without Landorus is just educated guessing until you see it definitively.
So, given Landorus' uniquely excessive power, restrictiveness on teambuilding and availability of better coverage than it has ever had before (in Knock Off, primarily), I am currently leaning towards banning Landorus-i from OU.