Yeah, a big mistake I see is when people want an unfit/retirement finding and they spend all this time in the NMA writing about what a stellar Sailor/Marine they are. Then, the board oftentimes thinks, "well, he's not that disabled, the commander says he is a great performer." I would not spend anytime making your self "sound good" on paper. It doesn't help you get a correct rating and so long as you are eligible for compensation (i.e., your condition was incurred/aggravated in the line of duty), they already assume you are "deserving" (in the sense that you aren't a dirtbag). On the other hand, I have seen many cases where such statements worked against the member.
It's more of an art than a science writing these things. Just make sure you cover all the information that the board might need to come to a correct rating.