I've bolded the things that can either outspeed Landorus and knock off its life orb, or the things that can (usually) take a hit and knock off the life orb, and have a reason to run knock off. Now, a few of these things, like Weavile, and possibly Azelf won't bother, as they can just kill it anyway with the appropriate move. Beedrill is too frail to take a hit, even after knocking off the life orb. I usually see diggersby with scarf, but it generally doesn't like to lock itself into knock off.
Now, you said that no one will let Landorus take a knock off before accomplishing its job (except for the people who call your bluff), which implies what? That they will switch out. You've forced the opponent to switch out of an unfavorable matchup and switch into either the appropriate check to one of the above, or a pivot, etc. When you do this, you cause the opponent to lose momentum, as you can play to them fleeing to avoid crippling landorus by making a switch yourself into a favorable match up, or set up (in the case of weavile, Mega Gallade, etc.) with swords dance. Now your opponent is at a disadvantage, they now have to deal with a +2 something, have to sacrifice something, and/or possibly even switch again (if you predict their switch correctly) giving you more opportunity to set up or deal chip damage.
What does this ultimately mean? The Landorus has cost you a lot of momentum and places you in jeopardy, now what?
Well, just like any wallbreaker, this risk can occur when you are forced out. Now, it was said that a pokemon should be uber if it can consistently create a disadvantageous situation by filling its role (in this case, it's wallbreaking) and is unreasonably overcentralizing, but as I just stated, it cannot consistently fill its role because it risks being defeated before it can break through something for its team.
When I built my semi-stall volt turn team, or whenever I build a team that's not HO, I do have Landorus in the back of my mind when I'm considering which pokemon to put on it; however, unlike greninja, I wasn't overly concerned about having several checks to it (crouching frog, hidden virtual duck2) as it has several checks that are B- and higher in the viability rankings. Now, outside of cresselia and Mega Latias, you're right in saying that it has no counters, but, Aegislash had counters, and it wasn't not overcentralizing. Mega Mawile had counters depending on its set, too, but that didn't stop it from being overcentralizing in XY (retest anyone?), yet, we look at the other end of the spectrum, Mega Garchomp has few counters at best (Togekiss? But Togekiss is steamrolled by stone edge, skarmory with Fire Blast, etc.) but, it's not overcentralizing. The point I'm trying to make is that having checks/counters is not proportional to being unhealthy.
I take issue with the bolded statements.
To threaten Landorus with the potential Knock Off, the KO user has to get into Landorus under circumstances that allow it to use the move before/without Landorus KOing them while he has the Life Orb. This would mean either something else fainted and they switched in for the Sac, they were brought in by Slow U-Turn and the pivot took a hit, or a fast U-Turn and they're risking the hit. And despite the utility and decent power of the move, I don't feel like Knock Off is quite as hard to switch into as a neutral attack from Landorus is. And you bring up the possibility of setting up or double switching, but that goes back to the prediction concept that has already been established to be a 2 way street. What if the Landorus player feels fighting a +2 Mega Gallade is more dangerous to them than Landorus losing his LO, so they stay in, call your response, and something gets smacked with Earth Power? Or hell, what if Landorus goes for Rock Polish on your set up turn?
252+ SpA Life Orb Sheer Force Landorus Earth Power vs. 0 HP / 4 SpD Mega Gallade: 220-261 (79.4 - 94.2%) -- 87.5% chance to OHKO after Stealth Rock and 1 layer of Spikes
He'll outspeed and you just lost your Mega, possibly your win condition at that. You can't assume the opponent is going to switch, because even ignoring the idea of predicting, they might simply think keeping Landorus in for a potential sac is their best option. Heck, it has to be considered you're risking a Pokemon as potent as SD Gallade or Scizor just for the possibility of threatening to cripple Landorus. Landorus always generates momentum by getting onto the field, and it takes some smart (and to an extent lucky) playing to get him off the field, whether that's bringing momentum back to your side or not.
In response to the last paragraph
- Aegislash's issue is that he lacked consistent counters: anything that countered one set was basically steamrolled by another, equally viable set he could run. Landorus is in a similar lane to a lesser degree, since knowing what checks him requires you to scout him and risk incurring significant damage either by luring or just by nature of the moves he can use.
- Mega Garchomp may lack defensive switch-ins, but he's plagued by other problems, such as a Fairy, Dragon, and 4x Ice weakness, lower speed tier (Landorus does get the jump on the still important Base 100's), and most notably requiring the Mega Slot, which hampers what Pokemon he can form cores with. Garchomp struggles with offensive pressure in trying to enter, and though he will kill something, he lacks tools like Landorus's boosting moves to allow him to 6-0 a team reliably.
Landorus has a low opportunity cost for having viability that equals or exceeds even the Mega Wallbreakers, which makes him ridiculously easy to put on teams and support things without requiring significant support himself.
232 SpA Pixilate Mega Gardevoir Hyper Voice vs. 0 HP / 0 SpD Manaphy: 226-267 (66.2 - 78.2%) -- guaranteed 2HKO
252 SpA Life Orb Sheer Force Landorus Earth Power vs. 0 HP / 0 SpD Manaphy: 230-270 (67.4 - 79.1%) -- guaranteed 2HKO
This thing hits harder than Gardevoir's Hyper Voice, generally considered one of the hardest mons for Stall to find switch ins to, and not only is Landorus easier to fit onto a team, but he has equal if not better coverage, the ability to boost his speed, or just annoy his checks with things like Knock Off, or even Stealth Rock/U-Turn if you want to try them.
The issue with Landorus to me
is just not a lack of checks and counters. It's the fact that Landorus's level of versatility and effectiveness in what he does makes
counter play an uphill battle by default. I have to make the sacrifices necessary to scout Landorus just so I can fight it on even grounds, because until I can be sure what it's running, I have to assume none of my checks will work lest I lose the one(s) that work(s) due to misreading the set, if I had effective answers in the first place. Fighting Landorus requires you to play smart and reactively just to reach the point where you could counter play it with potential prediction like you could Gengar or Keldeo or Metagross or most other Pokemon from turn 1.
