Brain field is powerful, but has some hints that there is a darker side to it. Are there any drawbacks (like being locked out of "good" endings) for using it?

Yeah, i was pretty bummed there wasn't more to the brain field part of the story

nope. there's only one ending, and it's the same regardless of who you play as (disappointing, I know). in case you haven't finished the game yet, I advise you to lower your expectations regarding the story. it's badly written and makes no sense. don't take it too seriously, think of it as a pretext to the awesome combat, and nothing more.

Brain Field is presented as some dramatic power boost, but in terms of actual gameplay it doesn't really do much. you won't get through an encounter much faster with Brain Field vs without using it, and depleting the crush gauge actually becomes more difficult. it's marginally useful once you get the invulnerability perk, and until then it's more hindrance than help.