Here's my stance with Comet and Aery:
Aery feels bad when there's no way to apply the ally shield. It really feels like a rune meant for a utility mage or an enchanter and many who are forced to use it do so knowing it's because it's the "only option".
Comet has a different problem, in that unless you have a way of applying a CC, many champs can avoid it too easily. And in other cases its damage feels lackluster especially when the enemy CAN avoid it so much. And so they take Aery instead, which then feels suboptimal because they can't use the ally shield.
The main problem is it really feels like there's no rune for mages with little/unreliable cc and no ally utility.
If anything, I'd like to see Comet be stronger and even harder to hit and another rune introduced for other mages with little/unrelaible cc.
Edit: I should complete my thoughts. Phase rush is not optimal on all mages, and this is not even in the running as a third option in my mind (not that I think it's bad, it's my fave rune on Syndra but it's definitely not for everyone)
Logically, you'd think "then try a different tree and keystone" but the problem is that Electrocute is the only other logical choice; after all, Thunderlord's Decree was a great option on a lot of mages, but a whopping 50 second cooldown early on makes the rune almost unusable and not worth it for what's lost in trade potential in lane. Which again, is fine, I like the new Electrocute as a jungling option since you'd only ever gank every 50 seconds or so and it feels appropriately powerful.
We just REALLY need another rune for mages. They, and bruisers who favored fervor, are just not getting catered to the way everyone else is.