I definitely do not think she is a poorly written character. In fact she is pretty well written in most regards.
But more so its my manner of perspective I grew up with Jaina being very kind hearted selfless person who wanted nothing more than red and blue to be a little purple. Then her city gets nuked by the controversial Garrosh and she flips from being that peaceful person into a hate raging woman who wants nothing more than the Horde to die out.
Its understandable I understand that part I am not saying it makes no sense she's forgiven the Horde right on the spot. But when you have characters like Baine, Thrall, and Vol'jin who are -trying- to make amends and you have Jaina say one thing and go to the next and then continue to ride that train of hate that's another thing right on its own. I can't tell if she's making baby steps or running backwards right into herself.
Her character makes sense to be a raging ball of hatred but that doesn't mean I like it - honestly I prefer the Jaina of old but I realize that Jaina died in Theramore (as so many claim she actually did and a Dreadlord took her place). I have love hate relationship with Jaina and I look forward to seeing what they do with her in BfA as well as many of the other main characters like Thrall, Sylvanas, Anduin, etc.