I think the skepticism some readers have expressed about Ezri as a captain is understandable, especially if one is considering her solely in the context of her Season Seven arc on DS9. Most people, after all, remember her as hesitant, awkward, and in over her head.
As far as I'm concerned, Dave did a great job with her as the older, more seasoned, more confident, more mature officer we see in Destiny
while still making her completely recognizable as Ezri. But for readers who aren't aware of the character's evolution over the course of the last twenty or so post-TV DS9 novels, or who may not be considering her innate potential as the ninth incarnation of Dax--with all the knowledge and experience that implies--then I can see how even the battlefield promotion described in Gods of Night
, five years after the end of the Dominion War, might seem like a strange creative choice.
But I think if one stops to consider all those things, taken as a whole, then Ezri's rise through the ranks isn't quite as odd as it may seem at first glance.
Your mileage may vary.