Well, one thing to say it's powerful. It depends on the context, really. Some people want to explain something they felt they couldn't while alive. A lot of shame related (justified or not) things come this way. In the event of being young and wanting an adult to read something it makes sense, I know a friend who's mother passed very young and she had a letter for her 21st bday, I think that was a lovely and great gift while I understand at times my friend had issues wanting to read it younger.
If it's a fuck you letter to anyone, I think it's sad and foolish. The writer has passed, you have no power over them so they "won" being hateful on your last breaths is some Moby Dick shit.
Lots of people leave something if they want to explain a reason or feeling at the end, be the end an illness or by their own hand, an attempt at closure. I don't think this is wrong.
I understand why people can and do feel it's a dick move at times because it's the passed party getting literally the last word and ending a talk before it can happen.
Personally, I only have my will for if something happens, and because I'm me there's a really bad joke in it, not dark just corny. I don't know if I could put on the words what to think or feel so I chose not to write anything myself, then again I'm sure hoping if I did that letter would collect a lot of dust.