I feel like the best Lando-T sets are the bulky SR setting pivot and the scarf set, as evidenced by all the calcs here, AV Landorus-T still can't switch in to a lot of special moves as most things outspeed it and can still 2hko and even if they can't they can just switch out and you're left predicting what they'll switch into with knockoff as your best option usually.
The fact that the pivot set is generally better doesn't make Assault Vest Landorus-T any less viable. They're two very different things. As I said earlier, the Assault Vest set functions not only as a special tank, but as a sort of lure for Pokemon that ordinarily blow right past it. Even if the opponent does switch out for fear of a Scarf set (which is expected, as this set does double as a Scarf bluff), Landorus-T has still done its job in preventing a sweep. As far as special tanking roles go, it switches into mixed LO Aegislash easily and avoids the 2HKO from any of its moves after Stealth Rock, which no other Landorus-T set can do. It switches in on offensive Heatran pretty effortlessly (bar HP Ice) and forces it out. In fact, it actually has a small chance to survive a 2HKO from Modest BalloonTran's HP Ice, a really good chance if it's Timid. The strongest of Mega Venusaur can only hope for a 4HKO (3.4% chance), while Earthquake easily nets the 3HKO. Physically defensive sets still lose 1-on-1 since Giga Drain is a 6HKO while Earthquake is at least a 4HKO. Landorus-T shuts down Gengar effortlessly; non-Life Orb sets have only a 8.8% chance to 4HKO with Shadow Ball, while Knock Off destroys them. Espeon (not sure why this is still used as much as it is, but whatever) gets shut down and destroyed. Landorus-T can take 2 of anything non-Specs Goodra can throw at it besides Ice Beam while 2HKOing with Earthquake, and if it can avoid switching directly in, it can even take an Ice Beam in a pinch and still KO. Landorus-T can take anything Life Orb Alakazam can throw at it besides Psyshock and HP Ice while OHKOing with several of its moves. Landorus-T is immune to Thunder Wave, and so with its great Assault Vest-boosted special bulk, it can easily shut down stupid paraflinch Togekiss (which is a big plus for me personally because I hate those things). It can survive 2 hits from mixed Infernape, while physically defensive sets will be 2HKO'd by Fire Blast or HP Ice. In a pinch, Landorus-T can take a Draco Meteor from Specs Latios before hitting back with Knock Off, which OHKOs after Stealth Rock. In fact, if you can keep Stealth Rock on the opponent's side of the field and keep it off of yours, Landorus-T will always win 1-on-1 if its switches directly into Draco Meteor since Latios will fail to 2HKO while Landorus-T OHKOs back. Landorus-T 2HKOs non-physically defensive Clefable with Earthquake while taking even a Life Orb Ice Beam from offensive tank sets. Even though it can't switch in directly, Landorus-T can take anything from a Life Orb Starmie (including Ice Beam) after Stealth Rock and usually OHKO with Knock Off. Mega Gardevoir only has a 9% chance to even 2HKO with Hyper Voice after Stealth Rock, while Landorus-T almost always OHKOs with Earthquake after Stealth Rock. Sheer Force Landorus without HP Ice gets shut down, especially after its Life Orb has been Knocked Off.
So yeah, Landorus-T is perfectly capable of taking special moves and actually beating specially offensive opponents. That initial list I made was merely to show how it does against some of the most dangerous hits it would have to take. LO Thundurus's HP Ice? LO Volcarona's +1 Fire Blast? LO Aegislash's Shadow Ball? Genesect's Ice Beam? Mega Lucario's +2 Flash Cannon? These are all very powerful and dangerous attacks for a Ground/Flying-type Pokemon to take, and it's impressive enough that it can actually tank those. Sure, it can't do it constantly throughout the course of a game, but then again, the pivot set can't tank physical hits forever either. It also still loses to a lot of common physically offensive Pokemon. SD Garchomp still 2HKOs with Outrage and survives anything Landorus-T can throw at it. Mega Charizard X need only toss out a Flare Blitz at first to scout the first Landorus-T switch-in, and the next time it comes it, it can KO after Landorus-T switches in on Dragon Dance and sweep right past it. Landorus-T without Stone Edge doesn't even beat Talonflame 1-on-1, the most common physical attacker in OU. In fact, it's only guaranteed to safely switch into CB Terrakion once, because if it comes in on Stone Edge the first time and forces Terrakion out, it'll be 2HKO'd the next time it tried to switch in. Now, I'm not trying to say that physically defensive Landorus-T is a bad physical tank, but it's not exactly going to completely shut down all of the metagame's best physical attackers.
The pivot set is still better more often than not due to the fact that it has access to Leftovers recovery and Stealth Rock, but it's not like Landorus-T is the only sturdy Stealth Rock user out there. Besides, directly comparing the two sets isn't really right because they both have their advantages and disadvantages, they both have Pokemon they shut down and Pokemon they still outright lose to, and they both play very differently. If you need a physical tank with Stealth Rock, you can go for the pivot set. If you already have Stealth Rock and need something to switch into things like Aegislash, Togekiss, and Gengar with ease, check things like Mega Lucario, Volcarona, Landorus, and Talonflame, provide solid offensive power, steal momentum with U-turn, and be a general nuisance with Knock Off, nothing does it quite like Assault Vest Landorus-T does.