I see this Suspect being quite similar to the last one. Deoxys and Landorus each have their main sets that are the reason they are being Suspected, as well as less common other options that get around counters while sacrificing some of their original purpose. There are two main differences between the two Suspects: what the Suspect sacrifices in order to beat its counters, and the cost of failing to prevent the Suspect from accomplishing its goal.
For example, in order for Deoxys to get around Forretress, it must either give up Magic Coat, Stealth Rocks, Spikes, or Taunt. Losing any one of these options means that a much larger group of Pokemon can prevent Deoxys from setting up its hazards. The opportunity cost of HP Fire Deoxys is very high. Now, take Sand Force Landorus. What do you lose? The ability to beat Skarmory, and other physical walls. What do you gain? The ability to beat most of your previous counters. With Deoxys, you traded beating one threat for losing to many others. With Landorus, you basically trade beating dedicated Physical walls for beating dedicated Special walls. You still beat any non-dedicated counter.
So to try and make this a little less confusing:
Deoxys-Defense @ Leftovers
Trait: Pressure
EVs: 252 SDef / 252 HP / 4 Def
Calm Nature
- Magic Coat
- Spikes
- Stealth Rock
- Taunt
Vs.
Deoxys-Defense @ Leftovers
Trait: Pressure
EVs: 156 SDef / 252 HP / 100 SAtk
Calm Nature
- Magic Coat
- Spikes
- Stealth Rock
- Hidden Power [Fire]
What you lose:
-Can not stop set up sweepers
-Must predict around with Magic Coat in order to stop Hazards being set on you
-A small amount of bulk
What you gain:
-You now beat a few previous counters
Now look at:
Landorus (M) @ Life Orb
Trait: Sheer Force
EVs: 252 Spd / 252 SAtk / 4 HP
Modest Nature
- Earth Power
- Focus Blast
- Hidden Power [Ice]
- Rock Polish
Vs.
Landorus (M) @ Life Orb
Trait: Sand Force
EVs: 252 Spd / 252 Atk / 4 HP
Jolly Nature
- Earthquake
- Stone Edge
- Superpower
- U-turn
What you gain:
-You now beat Celebi and many other previous counters to Sheer Force Landorus
What you lose:
-You lose to Skarmory, Hippowdon, and Lando-T
So clearly, much like the late Deoxys-D, Landorus-I can beat all of its counters while giving up much less of its utility.
My second point is just what each Pokemon can accomplish should your counter to it fail for whatever reason. If the Celebi you need to beat Lando dies somehow, it's probably game over right there. If your Choice Band Tyranitar you were planning on beating Deoxys with gets Superpowered by Deo on turn 1, chances are Deoxys will just set up 2-3 layers of hazards, a much more manageable situation that a +2 Sheer Force Landorus.
I hope my heavy comparisons to another Pokemon do not violate the thread guidelines, however I think using the precedent of Deoxys-D is a necessary argument for the banning of Landorus-I.