I think this is a really excellent point, and applies to more than just Brienne. The fact is that the TV show simply cannot give as much screentime to characters as do the books. The books can take their time with character development, revealing it bit by bit, etc. The show is necessarily going to have to compress/adjust some of that to get the characters where they need to be, while at the same time not developing characters by such dribs and drabs that it gets lost in the rest of the episode. I think this means that character develepment is going to come more in fits and starts, or perhaps zigs and zags, and perhaps earlier or later than it does in the books for each character.
I mean, think how few shows actually show development of character as opposed to just revelation of character. ASOIAF is so chock full of real character development over the course of the books (and within each book), that it would be perhaps an impossible task to do it on screen the same as the books. Because to show development, you first have to establish the initial character in the minds of the viewer, then have events unfold that make changes to that character appear reasonable. That must be an extraordinarily difficult thing to do properly given the sheer number of characters in ASOIAF.
So to the extent some characters appear to be a shade or two off from where they are supposed to be at this point -- and perhaps Arya, Brienne, and Danys would fit that -- it is entirely possible that more significant events in subsequent seasons will "zig" back to correct the "zags" that have happened so far. But at least at this point, I don't see any character so far off his/her character from the books that subsequent events cannot occur and still make sense.