I know a FeNe ESFJ who is ridiculously flaky in a cavalier way, to the point where I considered putting our friendship on hold until she matured and became more respectful.
There's something to this. Although it is said to be the domain of Ps, I have seen flakiness from Js, as well. They just go about it differently.
I don't think anyone who knows me IRL would consider me a flake and I'm an EP. They'd tell you, when asked, that I am a procrastinator, that I can be chaotic at times, that I have very poor time-management skills for social engagements, but I'm in no way, flakey with commitments I've made/promises that I've given. And if I do back out, it is due to a more urgent or critical matter coming up. "Sorry I can't make it to your birthday, even though I said I would be there, but my mom just went to the hospital tonight."
I've had this discussion with my close others, as I know that I'm highly impressionable. I do flake when it comes to my interests, and following through with pet projects that I've set for myself, where there are no consequences for others, as I don't have the best follow-through record. I can change my mind so fast, it can give others whiplash. However, if I change my mind, when it comes to a commitment that I've already made, then, the only times I go with my changed mind, rather than following through with the commitment is because I have a sound rationale for why the change of mind is more important than following through with the commitment. And, in such cases, I will call up the person I made the commitment to, and explain my rationale, which, most of the time, they understand, as there's a sound rationale behind it. Other times, it ends up that I convince them enough to change their mind as well, and they follow me with my new change of plans.
My ENFJ friend described me as such: "You're unpredictable but not unaccountable."
I think it's about responsibility and obligations. Not only a P versus J issue.
Js, in my expedrience, can, and do, manifest flakiness, in their own "J" way. Now, given the below are only anecdotes, but:
ESFJ 1 takes on so many responsibilities, commitments, as she likes organizing this, leading that, plus, her own work, classes, homelife, etc., etc., and then, inevitably, she gets stressed when they all come to a head, and starts to back out of one of her many commitments she has made because it's "too much". Flakey.
ESFJ 2 feels bad for saying "no" when asked to help out, and/or feels like it would be socially unacceptable to say "no" to the particular thing. Then, resentment and anxiety builds, and finally, they decide to do what they should have done in the first place. Say "no", but by then, it's coloured with expectations and having committed already, etc., etc. Turning a "yes" to a "no" is harder than starting off with a "no" in the first place. <- A thing I've told her many times. My INFP mother has this same issue. Flakey.
ENFJ aims to read what the other wants from them, trying to accomodate the other, but, in the end, realizes that the resultant agreed-upon decision wasn't a fair compromise between themselves and the other, and then starts to backpeddle. Flakey.