Robin doesn't really zone with projectiles in any kind of an effective way; the whole point of the Thoron is to force the mobility not to be such a crippling issue. If they're moving into your range, hit them with your sword. If they're staying out, charge Thoron. If they try to hang just barely outside of range or are using someone like Shulk who has a longer sword than you Arcfire them. I don't see the dead spot, especially since while Robin's mobility is pretty poor it isn't zero. I mean, Robin isn't fast; it's not easy. If you throw out the wrong move at the wrong time (or don't attack when you need to!) you get punished, and it requires good fundamental defense to escape bad situations. It is, however, doable, especially since so many of those aerials auto-cancel and you do have a pretty serviceable jab (you honestly don't even need your tilts/dash attack that much so their lack of safety isn't a huge deal). Yes resource limits make this even harder yet; you have to really play the spatial control game carefully and smart, but everything that matters recharges fast enough that it isn't crippling.
Emblem Lord I know plays many other fighters so perhaps this analogy won't be wasted. A good parallel character from another fighter is Dormammu from UMvC3. Dorm was honestly a pretty slow guy in a game that was mostly speedy rushdown, but he was one of the best characters in the game. He had a lot of projectiles but spamming them on average accomplished nothing and got you killed. It's not like he was a good support character either; his assists kinda sucked. How did he do it? Well, his normals were really good at stuffing approaches, his punishes hit hard, those non-spammable projectiles were at least really good at sniping people if he had a read, and if you elected to just hang back he'd charge spells which let him rain meteors on the battlefield which was as strong as it sounds. Charging spells in that fast paced of a game with people rushing you was pretty much impossible (there were a few sneaky set-ups mid-combo, totally predicated on you landing that hit in the first place of course). It didn't matter. The mere fact that you could forced people to play around that fact. No one ever wanted to give Dorm breathing room, and his mere presence on the battlefield forced people to play his game. His spells could be the most powerful move in the game you never used just because of what it forced his opponent to do. If you actually did get your rushdown in on him his escapes weren't even all that great, but he at least had high health and was able to use the universal defensive options just like everyone else to endure. That's pretty much Robin except with Thoron instead of Dorm's spells. You can play a full game, never hit the neutral special once, and Thoron can win you the game.
Robin is probably not as good as Dorm was, but that fundamental gameplan is sound. It's hard; Robin is probably one of the hardest characters to play since you have to be so careful between your difficulties at point blank, your low mobility, and your resource limits. It is, however, very sound, and I have a firm belief its power will only grow with time. Now obviously I'm talking out of pure game theory here; this game is way too young for us to actually be sure which characters are good or bad especially complicated characters like Robin, and Robin isn't even popular locally so it's not like I even see an awful lot of him/her. I just can't help but look at the tools I see and see a powerful character, and it's easy not to worry about early difficulties since Robin is transparently a very difficult character to play (but difficult to play does not necessarily mean bad).
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ParanoidDrone
Robin's only aerial that doesn't AC out of a shorthop is dair. Dark Fists is also just so scary on ledges; it's hard to ledge trap Ganon when he can throw out that kind of a hit at whim.
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Conda
I dunno; I think that's biases from old smash games talking. All of the past smash games had characters who were good at everything that mattered alongside several specialist level strengths (64 Pikachu, Melee Sheik, Brawl MK), and that distorted the metagames by making a lot of specialists just obsolete with the further question of why play a specialist at all when you can have specialist level strengths with no weaknesses of significance at all. A lot of other fighters have avoided that problem over the years, and I think 4 avoids it for the first time in smash series history. In these other fighters, sure the well rounded Ryu types always do well, but the specialists who are supposed to run into problems just overcome. Just look at least year's EVO and how well Snakeyes placed in SFIV with Zangief, a super extreme character who I'm pretty sure allegedly has several hard counters. While I would agree that high tier "well rounded" characters are safer and easier, I'm not really convinced they're better than the more extreme guys; often the extreme characters require more creative thinking in certain match-ups but can make it work and then proceed to ruin everyone's day when that happens.