Having "dabbled" in the online hypnosis scene for a while--my account on WarpMyMind (WMM) dates back to 2008!--I've reached these conclusions as they relate to NBE:
1. Hypnosis is not magic.
The idea of trancing to an audio file or spiral or whathaveyou to grow breasts is highly appealing. However, ultimately, breast development and growth is a biological process. It's not possible for a hypnosis script to overrule biology. (There's a file on WMM that says it'll turn your penis into a vagina. That's impossible.) This doesn't necessarily put the nail in the coffin for NBE, although it does eliminate the practical usefulness of files that suggest non-realistic results (such as "Curse Breast Growth" which says your breasts will never stop growing).
2. Theoretically I think NBE is possible through hypnosis...
...though, I have yet to see anyone post any compelling results on the WMM forums. Basically, hypnosis affects the mind, especially on a subconscious level. The question then is whether the mind has any control over that pesky stuff that makes us physically male or female, ie hormones. My gut feeling is that it does--after all, hormones and their regulation stem from the brain. And the male body does have estrogen, it's just the levels of estrogen and testosterone that need tweaked.
3. However, theoretical possibility doesn't necessarily translate into anything tangible.
There are a lot of things our bodies do automatically. We receive pain signals when we stub our toe. We flinch when we see something fly towards our face (even if we know logically it won't hit us, like seeing something on a movie screen). Our hearts beat. We breath. Etc. Some of these things are possible to override. It's easy to hold our breath, for example. After years of practice, people who engage in deep meditation can slow down their heartbeat. So the question becomes--IF hormone levels can be nudged on a subconscious level (and there's no guarantee even of that--I'm flying totally on gut feeling here), how long would the subconscious mind have to train with a hypnosis file to reach a point of being able to do that? Months? Years? And that's assuming you can go deep enough into trance in the first place.
And all of that is assuming the file you're using is well written enough to be using suggestions that your subconscious mind will even accept.
4. That doesn't make these files useless.
While breast growth hypnosis files (for men, anyway--women would likely have an easier time getting results from something their body has already naturally done) aren't the greatest path as a single method for NBE, they do provide several useful features: 1. Giving the user the ability to explore the concepts of breast growth (via visualization/fantasy/conceptualization) without the fiscal and actual-factual commitment of taking hormones. 2. Transitions (whether actually taking the full trans* plunge, or just lightly feminizing for whatever purpose) go smoother on a mental, emotional, and maybe even physical level if you've got a positive, open, and ready mindset, something these files are (typically) pretty good at fostering.
5. There are a TON of feminization/trans*-oriented files that aren't about the physical transformation.
Hypnosis can be very effective with anything that has to do with "inner" changes. Files can create new mindsets, perspectives, desires, interests, compulsions. There's likely a file on WMM or elsewhere that will promote feminizing in a way that appeals to you. However, it's always wise to vet a file's suggestions before trancing to it to make sure all of the suggestions within it are something you're comfortable with. Hypnosis is a highly fetishistic arena, especially feminization/trans* stuff. Themes of humiliation/femdom/bimboification/submission/bisexuality and homosexuality/etc are commonly interlaced with feminization files.
Feel free to message me if you have any questions, want file suggestions, or would like a WarpMyMind crash course.