I'm just gonna go ahead and list the positives
and negatives of Landorus's two Formes.
Landorus-Incarnate
Positives
- Seriously powerful thanks to Sheer Force and 115 Special Attack
- Has exactly the movepool to abuse Ability and stat with
- Large coverage with its Sheer Force-boosted movepool
- 101 Speed barely outpaces a majority of the metagame
- Can even run mixed sets with Knock Off or (somewhat niche) physical Sand Force sets
- Stall is terrified of it
Negatives
- 101 Speed isn't
that great and leaves Landorus-I outsped by a large slew of Pokémon
- Rather predictable compared to Therian
- Has somewhat of a case of 4MSS, as Jukain explained
Landorus-Therian
Positives
- Fantastic 145 Attack stat gives it amazing physical firepower
- Intimidate gives it great initial physical bulk and forces many switches
- Combination of U-turn and Intimidate makes it a great scout (Scarf) or pivot (defensive/SR)
- Can run a variety of sets to great effect, making Landorus-T highly unpredictable and versatile
- Has a shitton of ways to cripple or get past its would-be checks and counters
- Typing is both offensively and defensively fantastic and goes greatly with its stats and Ability
- One of the best switch-ins to Sand Offense
- Checks just so fucking much in the current metagame, can do a ton of shit and is extremely versatile in general
Negatives
- 91 Speed is kinda low, so as a Scarfer, it's prone to being outsped by other Scarfers
- Worn down somewhat quickly due to lack of recovery
- Doesn't like to be locked into Earthquake or Superpower
Now, as you can see, both Landorus Formes have a lot of positive traits that more than make up for their negatives. There's not just an opportunity cost for using Lando-T over Lando-I;
there's either an opportunity cost for using either or there is none at all. Let's be honest, both have about an equal opportunity cost. Using Incarnate means you'll have to miss out on Therian's utility, but using Therian means you're gonna need a replacement for Incarnate's wallbreaking prowess.
Going on about this point; there's usually an alternative for Incarnate in the form of Mega Charizard Y or Mega Gardevoir, the latter of which Therian pairs extremely well with. Both are extremely easy to slap on teams and provide a lot of synergy, but overall I personally think the current metagame is more in Therian's favor than it is in Incarnate's favor. Many Pokémon highly appreciate Therian's scouting/pivoting ability in the current metagame.
FINAL VERDICT
Both Landorus-Incarnate and Landorus-Therian have their pros and cons, the choice between the Forme all depends on what the team requires.
Both are highly deserving of S-Rank, but for different reasons. Neither is perfect, but they're both fucking amazing.
Done talking about it. Sayonara.