If you ask a lot of competitive players what the single best Pokemon in its given metagame is, a lot of them will answer Landorus-Therian. A staple of OU since its inception, it doesn't take a genius to see where Landorus's success comes from in either form. Both forms are blessed with fantastic Ground/Flying typing, good overall bulk, and great (and efficient) coverage. Its Therian form has Intimidate, one of the most useful utility abilities in the game, and a fantastic base 145 Attack stat. This combined with its naturally decent bulk, great typing, and access to many important support moves like U-Turn, Stealth Rock, and Knock Off, makes Landorus-T pretty much the premiere pivot for Singles, and allowed it to top the viability ranking charts for 3 and a half generations now.
Landorus-Incarnate takes a slightly different approach than its Therian form. With its somewhat lower offensive stats, you might, at a glance, expect it to pack less of a punch than Landorus-T. You'd be wrong. Sheer Force makes up for the slight difference in stats, turning Landorus-I into a frightening breaker that is incredibly difficult to switch into, and the fact that it still has a usable Attack stat backed up by moves like Knock Off and Superpower prevents dedicated Special walls like Chansey from switching in with impunity. It also crucially sits just above the base 100 Speed mark at 101, where Landorus-T sits slightly below it at 91. That 10 point difference might not seem like much, but there are a ton of Pokemon that sit between it that Landorus-I can outspeed while Landorus-T cannot.
Both Landorus forms are highly customizable Pokemon that can run a huge number of different sets, with many variations on those sets to deal with specific threats. This is especially true for Landorus-T in unofficial formats; the "standard" Landorus-T set shifts so often to keep up with the metagame (which in turn shifts around it) that trying to keep up with it means analyzing usage statistics for the high ladder and high-level tour games. It can sometimes be difficult to play around Landorus until you've identified its set; assuming that your opponent is using a defensive set as a bulky pivot can be disastrous if it turns out to be Scarf, for example, and unlike many other Pokemon there really isn't a safe assumption to make either way.
I'm not really a Doubles player, but with a strong Earthquake and Rock Slide and access to Intimidate, one of the best abilities in Doubles, I understand that Landorus-Therian made quite an impact in VGC as well.
As for what holds it back, Landorus famously lacks a reliable Flying STAB, having access only to Fly. This prevents it from leveraging its Flying typing offensively. It also lacks recovery, which can leave it prone to getting worn down if it's used to check multiple different Pokemon over the course of a match, and its bulk, while decent enough especially when bolstered by Intimidate, isn't amazing by any means, which forces it to rely on offensive pressure to deal with many Pokemon instead of just walling them indefinitely the way a bulkier defensive Pokemon might be able to do. Despite all of these flaws, Landorus is still one of the best Pokemon in the entire game.
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