Sure, maybe management did make some questionable decisions, and perhaps the PI was especially tough to work for. But a resignation letter is probably not the best place to bring that up, Suzanne Lucas notes at her Evil HR Lady blog.
"Exit interviews and resignation letters should be thought of as marketing documents just as much as your résumé is," Lucas says. "If you leave in a spectacular way, telling everyone how rotten your boss is, it will get around." Plus, she adds, by providing negative feedback at this stage, "you have nothing to gain … and plenty to lose."
While it might feel great, airing out unfavorable feelings about a job while leaving it will likely do more harm than good, Lucas says. "Exit interviews can mention non-confrontational things like opportunity/more money/returning to school/changing industries," Lucas says, "but not bad managers, unfair practices, and how you never got to use vacation days."