First of all, let's see from where this came from. Offlane solo is probably the hardest role in a game. Offlaners rarely take farm, and often get below solo experience. There are some important quality that an hero have to possess in order to be usable as an offlaner:
- he need to be able to work on little farm
- he need to be able to avoid death
The second point being the most crucial one. Offlane solos also often need exp much more than they need gold.
6.76 came running at you
With the advent of 6.76, magnus broke into the scene, and teams instantly recognized him as a strong offlaner because of what is essentially a magic immunity spell (immune to every stun except Disruption and Telekinesis iirc) that move you 1200 range away from danger. This made for an almost impossible to kill magnus, and made for the perfect offlaner, Magnus being an hero that worked fine on low farm, with just a dagger and arcane boots. More importantly, having an unkillable hero on the offlane made for a good laning phase, as you could, at least in theory, shut down their offlane while they couldn't shut down yours.
Needless to be said, the reign of 6.76 magnus was pretty short. The hero was immediately recognized and gained perma-ban status, while shortly thereafter 6.77 brought forth a skewer range nerf at lower levels. Magnus was no longer an untouchable offlaner, and the hero, while remaining a top pick (and probably the best one imho), is no longer a perma-ban.
Who ya' looking at?
While the adventure of the unkillable offlane Magnus was short, it was not without impact. Dota players recognized once again how strong unkillable offlaners were, even if melee and apparently in need of items (Magnus has a steroid after all).
I believe people started considering offlane Na'ix at this point. Na'ix has a lot of the properties that an offlane should have. He's extremely hard to kill, thanks to an instant cast magic immunity (allowing you to dodge a stun easily), high hp and base movement speed. He don't need a lot of items to kill you fast in the midgame either, as phase boots + Jjango are usually more than enough at that point combined with Rage, Feast, Infest and an amazingly high base damage. Even better, he can farm the jungle relatively safely after a few levels if in need of farm after the laning phase.
Juggernaut is similar, having high base ms, and being basically unkillable even at lvl 1 due to the 5 seconds instant cast magic immunity. He, too, work incredibly well even low farmed in the midgame, thanks to his spin, ult, and healing ward. He goes for a similar itembuild as Na'ix, prioritizing Movespeed in the midgame to keep its target under attack range in that phase of the game where there are still no forces around. In the IG game, Juggernaut was played as a mean to get kills in lane due to his strong tri-lane presence, but was not played as the main farmer (that being Luna).
Another important thing is to keep in mind how easily those heroes are countered if they go full-carry style. Na'ix and Jugger both have low mobility and basically no mean to counter a ghost scepter or a force staff heavy team (yes na'ix can infest a blinker, but that's still on a 100sec cd). This is why i think the 2 or 3 position is ideal for them. By playing them as a secondary DPS dealer and as non-conventional-initiator, the opponent will not find it worth to expend a lot of gold to counter a non-1 position hero.
I'd like to also point out which kind of main carry LGD.Int went for when it went for Na'ix. Luna in the first game, and SF in the third. Both have got massive magic damage that make a ghost scepter even less desirable and mostly a non-choice here. The same is true, even if less important because of Jugg spin, of IG vs LGD game 2 where they picked Juggernaut and Luna together, again making Ghost a risky proposition.
Problems, and why can't we live without them
Obviously, those picks are not flawless, and have risks. For Jugg the biggest problem is clear: the lategame. While Na'ix scale nicely with some items, Jugg does not.
The issue is the spin. Na'ix Magic immunity allows him to attack, while Jugg does not. This mean that you need BkB on Juggernaut. But even with BkB, you don't deal a lot of damage because you aren't the primary farmer, have little ways to farm even if you are, and you can't keep people from kiting you. Juggernaut is extremely weak in the lategame because of this, and is strictly a midgame hero that should be played when you can push and capitalize your early advantages, especially with his healing ward that can counter even the biggest antipushers like KotL. This is sort of why YYF went for an Aghanim. He recognized that going for damage items wouldn't have made him more of a threat because he didn't have a BkB, and he recognized he'd need too many items after a BkB to be effective in a fight, and instead focused on having an ulti at every teamfight since omnislash is much harder to counter than a melee hero with no means to keep people under his range.
Na'ix weaknesses aren't as clear-cut, but its more of a series of them. I believe the primary issue is its laning. While it's true that Na'ix work as an offlaner, he isn't as good as Jugg at avoiding death at level 1 because he only have 3 seconds of magic immunity, neither he is good at dealing inordinate huge quantities of damage in trilanes, hence why he's probably better off as a solo (faster levels are more important for him too, as Rage get incredibly better with levels). He isn't as good as pusher as Jugg, either.
Both have also some common issues. They are melee, they can't gank, they don't have stuns, and they won't go for the common items of offlaners (Arcane, Meka), meaning your team will have to make up for it. In those case, you want a meka holder that can farm it in reasonable times, especially because you went for an offlane that work best at 15 min in the game. There aren't many, mostly Chen, Magnus (afaik the only commonly picked mid hero that go Meka) and Enigma. Unsurprisingly, LGD.Int had Chen in both games they went for Na'ix, while they avoided Na'ix when they couldn't get Chen and they got an ancient farming Tinker for offlane (and i assume they aren't comfortable with Jungle enigma as it isn't a good ganker at level 1-3, so they couldn't double roam with him).
IG on the other hand, went for a mid Magnus in the game they had Juggernaut (Magnus working amazingly with Jugg, but Magnus work amazingly with pretty much everyone), and needlessly to say, Magnus went for a fast Meka that game.
The past and the future, they actually are the same
This section is just a rant of mine. As many of you may know, Omniknight was actually sometimes used as an offlane solo, for similar reasons as above: longass magic immunity. He also benefit greatly from levels, so solo offlane exp is great as long as you don't die. However, he has a pretty big problem: while he has the longest magic immunity skill in the game, it isn't instant cast, meaning you can't dodge anything with it, especially faster (or instant) projectiles.
This mean farming is way harder, and that is a huge problem when compared to Na'ix (that has huge base damage and instant magic immunity) or Juggernaut (that has a spin that deal huge damage in trilanes). He can't be used in trilanes either because he do nothing in those. He is, however, amazing after level 5 until the end of the game and has probably the best non-ulti skill there is (repel).
I think that he will probably be explored again a bit, because of how amazing his ability to reverse teamfights is. He remain amazing in pubs as an offlane solo because of the fact that people can't correctly coordinate chainstuns and will almost always allow you to repel to safety.