I think Landorus is clearly a worthy suspect. It has advantages that other breakers simply do not: Manaphy has no immediate power whatsoever, Mega Gardevoir takes up a mega slot, and Gengar wears itself very quickly. Landorus has immediate power, diverse set-up options, doesn't take up a mega slot, isn't choice-locked, etc. It's a notch above the Pokémon I mentioned above for those reasons.
That being said, this Pokémon's reputation has started to exceed its actual performance, in my opinion. Any set lacking Rock Polish is pressured a ton by faster teams. In this matchup, it is further hampered by how, in the same way that Aegislash's Shadow Ball is spammable, Earth Power is ridiculously unspammable. Yes, prediction goes both ways, but this is about consistency, and consistency matters. Everyone is weak to Mega Gardevoir on paper too, but it too can be played around. It also provides VERY limited defensive utility beyond sponging choice-locked Volt Switches. In a meta that can be as restrictive as ORAS is in team-building, this can be a liability.
"But Rock Polish." If it runs Rock Polish, it yields tremendous wall-breaking capacity--every three-move combination has a major inadequacy--and I don't think RP Lando is even remotely as threatening to offense as SD Sand Rush Excadrill or RP Mega Diancie/Mega Metagross. Its versatility is annoying, but you aren't dealing with Mega Charizard. Posts like James' emphasize how Landorus picks its counters and its advantageous matchups, but I find the inverse to be equally true: the metagame dictates how effective Landorus is, much like Mega Pinsir in XY. I don't think it's a coincidence that a bulkier metagame that has adapted to the presence of Pokémon like Mega Metagross, Mega Diancie, and Mega Altaria suddenly has trouble dealing with the best punisher of passivity in the tier.
For all the drawbacks Landorus can bring to team-building and for how match-up based its individual sets can be, its good matchups are freaking good and passive teams can really struggle against it, and usually do. I've always kind of hated Landorus' presence in the metagame for this reason.
If I had to vote, I would probably say no ban. I don't think Landorus is healthy for the metagame, but I also don't think it's broken...maybe. I don't have great reasoning for you--it's just an "eye test" thing. My vote might change when I hop on the suspect ladder, and I should have time to get reqs this time.
Also, what Syn said is 100% right. Not that it means that a ban would be wrong (see: Mega Mawile), but it's not like Landorus has changed at all since, well, the beginning of XY. (Of course, the metagame has, but I wonder if the potential inability of the metagame to adapt has more to do with the external force of the new megas than it does Landorus, as XY adapted pretty well to Landorus...)