would it make sense to provide honest (negative) feedback explaining the reasons for leaving?

I can imagine a case where it benefits the company. (Although if the company were as bad as you indicated, I don't imagine they would be open to constructive criticism anyway).

What I can't see is how it can benefit the departing employee in any way.

I'd still advise to give only neutral feedback.

Any feedback you could provide on the way out could be gotten through existing employees, if the company every really wanted to know.

would it make sense to provide honest (negative) feedback explaining the reasons for leaving?

I can imagine a case where it benefits the company. (Although if the company were as bad as you indicated, I don't imagine they would be open to constructive criticism anyway).

What I can't see is how it can benefit the departing employee in any way.

I'd still advise to give only neutral feedback.

Any feedback you could provide on the way out could be gotten through existing employees, if the company ever really wanted to know.