I'm resigning for ergonomic reasons and ideally I want to explain this at the exit interview, so they know it's nothing personal, and hopefully they will take action for others if they see that someone is actually leaving over it. Since it's nothing personal, is it professionally safe to explain my ergonomic reasons or should I just keep it vague like "it's not a good fit"?
Why would it not be professional to tell them the reasons you're leaving?
Yes, tell them.
There's nothing to gain by not being straightfoward with them.
There's nothing to lose by being straightforward with them.