The problem with scouting Landorus is of a similar nature (though maybe not magnitude) to what was up with scouting Aegislash: Landorus's different sets are generally optimized for different things without losing effectiveness against his "consistent" victims. In the time it takes to scout Landorus, he is very capable of doing serious damage to your team and check, even if it happens to be the right check.
More to the point, this would mean having at minimum 2 things that could check Landorus: One that checks him for all but one or two moves/sets (Say, Tornadus-T checks sets without KO or Rock Slide), and then something that can check him for those two options (say, Magnet Rise Klefki? This is a very hasty example so I can continue my point). So that's two checks needed. This doesn't seem like the biggest demand considering things like Altaria and even Metagross are also best handled like this, but Landorus is not only much easier to throw on a team, but on top of having a relatively small pool of checks, only Stall immediately has multiple fitting Pokemon that could check his sets without fearing similar options (M Latias/Cresselia + SpD Zapdos, for instance): offense has Tornadus-T and the Latis, but both fear Knock Off (not to mention the former hates Rock Slide); Balance has trouble with mix and matching these types of answers since the defensive answers tend to be passive, but they can't stack offensive answers since they need utility mons and/or defensive backbone of some kind.
Landorus's pool of checks is too sparse and unevenly distributed to allow any playstyle but stall to answer him completely and consistently; any other just has to hope he doesn't get the momentum he needs to wear out the right check or sweep right through the wrong one, which is a tall order considering Landorus requires relatively little support to do his given job.
Landorus has the potential to beat all of its checks (besides two), but can not beat all of them at once and is easy to revenge kill: A lot of people like to say that Landorus is too good versus every style, and that it can sweep and wallbreak at the same time, but this is just untrue. While Rock Polish sets have the potential to sweep, using this move makes Landorus a much worse wallbreaker. Limiting it to only three attacking moves greatly hinders its breaking capabilities, and allows many checks to be consistent. In addition to this, Rock Polish does not even beat offense that easily. Firstly, Landorus does not have many opportunities to set up versus offense. Many common offensive pokemon, such as Keldeo, Weavile, Thundurus, and Latios, all easily prevent set up and revenge kill Landorus. Saying that Landorus is anything better than mediocre versus offense is just untrue. Landorus does not have the raw speed or bulk to withstand offensive threats. As far as wallbreaking goes, Landorus is somewhat overrated. Keep in mind that this is not to say that Landorus is not a great wallbreaker. It is true that Landorus has fantastic raw power and coverage, but people often mistake potential success for actual success. There are plenty of prevalent pokemon that naturally defensively check many Landorus sets, such as Chansey, Gliscor, Spdef Gyara, Lati@s, Skarmory, Slowking, Torn-T, Cresselia, and other less common ones.
Sometimes scouting is just unnecessary because a team could have more than one Landorus check, and thus will likely not lose to a single set. This leads me to my next point.
My issue here is the fact that while you did denote these as defensive (though I think the Latis and Torn-T are offensive moreso), every check you listed bar those I noted are either incredibly slow, relatively passive, and prone to wearing out/lack reliable recovery. And Landorus isn't even working in a vacuum. You note it's unnecessary to scout just because the team has more than one Landorus check. Ignoring my query above about multiple Landorus checks that don't stack the same coverage weakness, Landorus isn't hard to support. Pursuit trapping was one example, but another thing to note is that your entire list bar Cresselia (the most passive example) can be beaten by, say, mixed Thundurus for example, and Double Genie cores have indeed been a thing since XY. So the opponent can stack as many Landorus checks as they choose, and I can still get by them with one mon for support, if I even need that much depending on my Landorus set.
Landorus isn't as hard pressed to get Rock Polish off as you make him out to be. While not walling anything, his bulk does require a good offensive hit to OHKO him, usually a SE one at that
252+ Atk Life Orb Excadrill Iron Head vs. 0 HP / 4 Def Landorus: 211-250 (66.1 - 78.3%) -- guaranteed 2HKO
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 0 HP / 4 Def Landorus: 232-274 (72.7 - 85.8%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Pixilate Mega Altaria Return vs. 0 HP / 4 Def Landorus: 232-274 (72.7 - 85.8%) -- guaranteed 2HKO after Stealth Rock
And Landorus only needs to survive that one hit to get his boost and sweep through the team in the mid-late game when he goes to work. I noted earlier Landorus's SpA with Sheer Force + LO is the equivalent of invested Base 228, which is higher than any wallbreaking Mega. However, that was with a neutral nature. Rock Polish sets run Positive offense natures, which actually means they will hit harder than most sets without set up.
The result of this is that Landorus's offense match-up set doesn't sacrifice his match up with balance/stall: If the opponent lacks a check to the 3 Attack set you chose, it might make him better since he's dishing harder blows and defensive mons aren't fast enough for the drop from Modest to impact his speed (which is 1 point below Jolly Excadrill, still decent for wallbreaking). I think the reason Rock Polish is more common than CM is because unlike CM, Rock Polish doesn't immediately trail behind the other sets against its non-favored playstyle, but rather has potential to perform as well if not better. It's not even like Landorus matchs up poorly against offense without Rock Polish, just that Rock Polish is his best set against it; without it, the slower wallbreakers still have to stomach the strong blows since the faster members can't switch in but have to be ready to revenge. It just means Landorus won't be sweeping through the team, which isn't even necessarily his job in the first place: the RP set CAN do that as another benefit, but I have had games where it did not need to do that to be a major factor in my win.