I think there are some weaknesses that should be acknowledged with Grace.
First, Grace is susceptible to kiting, though we already know this. Nothing more is to be said or explained.
Second, Mortal-Wounds in a Spellfire-Heavy Meta severely reduces her healing potential.
Third- Grace also does not like tanking poke. She gets worn down easily with the likes of Celeste and Skaarf when they play it safe and poke from afar, and Grace is unable to reach them. This means that she has no form of sustain unlike someone like Ardan, Lyra, Churnwalker, etc. (She can heal herself, but with such long cooldown to her ult, it is better if she uses it on her allies instead.
Finally, Grace can’t protect her carry as well as some captains can. Once the shield duration is down for her allies, they become squishy again and the enemy team can easily focus on the squishy on Grace’s team and eliminate him/her and Grace can’t do anything but cast her ult and become useless for 1-2 seconds in a fight.Her stun isn’t a kind that can be used to stop the enemy from killing her teammates as she can’t easily retreat once she dives in, as when she dives in, she has to commit to a fight as she has no escape. Lyra has a speed boost, burst heal, and a bulwark that stops enemies, not to mention her portal(though it can go both ways). Ardan has his gauntlet and his vanguard. Catherine can bodyblock with no fear for being bursted down for 3-4 seconds and stun diving enemies.