Basically, her design is like... really counter-intuitive?
She's so reliant on reacting defensively / counter-offensively due to her slow speed and frame-perfect precise aerial game, yet they made it so that she doesn't have many safe tools to force any of these situations where she shines most.
Din's is completely out of the question most of the time, especially on anyone remotely fast mobility wise (you Din's on Sonic or Falcon, you're just asking to get brutally murdered).
Phantom Knight is alright, but there's still a significant amount of commitment you need to make to it in order for it to be remotely effective (better than Din's by a miles though). There are times where if you don't let go of Phantom crazy early as they're approaching you from the air let's say, it's a free punish for them with how laggy it is after words.
Not to mention that both of these projectiles require a very obvious and predictable channeling time. Couple that with her slow speed and average reach, she can't really force a lot of things to happen with her regular attacks either.
It's ironic but, her worst match ups are probably when people play patiently and defensive. Against someone like Fox as an example, who can just spam lasers at you until you very predictably approach him, it becomes a very up hill battle for her as she has no quick reponse to "jab / needling" tactics nor does she have the speed to properly punish people for this.
You either have to get very risky, which is not very good, or you have to have a hell lot more patience, which can be discouraging against certain opponents that can easily dance and bait her into doing things because they have the safe tools to do so.
God, if Zelda has something like Fox's lasers or Sheik's needles or even a store charge Phantom Knight, I would be pretty happy about that as those are things that would actually force people to approach her since she would actually be at an advantage (more or less) in a neutral situation, which would alleviate (even a little bit) her glaring mobility flaws and be helpful to her character as a whole in my opinion.
Just my two cents of course, heh heh.