[Cyberis]

I'd make my resignation letter quite minimal. It will likely go in a file that will outlast the context of your time at the company and reason for leaving. Thus whatever you say there will have an increasing chance of coming back to bite you later. Your exit interview may or may not become part of your permanent record but you can convey better context in the interview. I'd still be careful to think about how your statements would be viewed in 1 to 5 years after they are given. I think it's better to err on the side of discretion.