I actually feel kind of the same way here. I really, really want to play Robin, and have devoted the 2nd most time to him/her (behind Megaman), but he/she's currently around my 4th best character. I say this fully acknowledging that it might be a personal struggle (which is why I came to this board.... only to see this thread).
So a few of the struggles:
-Mobility issues that everybody knows. I actually don't think this is a huge deal overall, but it adds to the struggle.
-Blind Spots in attacks. This is actually my #1 issue currently. Most of Robin's attacks have a pretty substantial blind spot where they just don't hit the opponent. Fair has a great hitbox, but not great enough to SH fair and hit short characters (without a laggy landing). Dsmash has a slow backward hitting hitbox (but it's not very useful/powerful and only occurs with the Levin sword), but we really don't have an attack that hits forward and back (which is invaluable in punishing rolls). I suppose you could count SH Nair here, but again, it has its limits. Elfire has a huge blindspot in front of you... and considering how slow it is... it's really not safe to cast it at the distance that it will actually hit somebody against most faster chars... so you essentially have to use it as a wall, run up and do it again (again not a huge issue, but it's a limitation when you only have like 6 uses of it).
-Issues against short characters. This is kind of relating to the point above but dang, we have a hard time just hitting the shorties in this game. All of our aerials lag like crazy when you start them low enough to actually hit people like Ness, Mac, Kirby, etc. This extends even to Thoron, which flat out misses (or can be ducked) against way too many chars. So combining those points, the short + fast characters (Mac, Greninja, Sheik all fall into this) just give us nightmares. We have nearly nothing confirmable in the neutral, so we just have to throw out hitboxes and hope the opponents get hit. I don't like that kind of passive gameplay, personally.
-I actually really like our jabs, but the strong third hits just aren't confirmed ever.
I do agree that platforms help us in general, so For Glory is not a great representative.
I do think Robin has more issues punishing rolls than most of the cast... Again, you can either commit to punishing the spot dodge OR the roll... nearly nothing can punish both of them with any power behind it (might be able to get a tilt or jab). This feels really limiting in a game where nearly every character has SOMETHING that hits forward and back (see Lucario dsmash for probably the best example)
I think if anything is going to turn this around, it's going to be using the books/levin sword as items. I do see some potential there, esp considering their power. I've gotten to the point where I'm not afraid to spam the sword a bit just to get the item. I also really like Nosferatu as a very good shield punisher + equalizer at high %s (although if you miss, you're toast)
I also want to point out the custom moves, in case they are legal. Robin is one of the only characters that I feel becomes quite a bit better once customs are active. In particular, I think his Fast Thunder and short-range Elfire (I forget the actual names and numbers but you'll find them) are VERY GOOD.
-Fast thunder allows us to essentially use Elthunder as our quick spam option (very brief charging). Given the size and usefulness, I think that makes it vastly superior to normal thunder. I also don't really think Thoron is a great killmove as is (and again it whiffs a lot), so the lesser power really doesn't bother me here, in exchange for a much faster spammable move.
-The Elfire special puts the bonfire RIGHT IN FRONT OF YOU and hits once powerfully instead of the multihit. Now I like the setups that normal Elfire gives us, but those aren't guaranteed anyway. This move also eliminates the blindspot that I mentioned before, and seems a bit faster. It helps us get down from the air because of it's very steep angle (which doesn't seem to disappear from any height before hitting the ground). Its main use, imo, is in edgeguarding though... any chars that like to go low (which seems like 75% of the cast), you can just stand on stage and hit them with this out of their upB. it's really really good.
I think the upB specials are interchangeable, but I prefer the normal one.
Same with the DownB specials... I think they all have uses, but the normal is my favorite.