While walking on eggshells:
>> I've said "it was just time to move on to something new" about a dozen times in the last one I did
Possible outcomes being honest in an exit interview:
- You help the company you're leaving improve its processes and culture
- You burn bridges by hurting feelings of people in your industry
- You may harm the chances of a rehire
- You may harm the chances of a bad reference
Out of that list, the only way to get the first outcome is to create a culture of good will where the employee WANTS to help or feels safe enough to help without fearing reprisal.
It's not easy to accomplish and in most instances people will err on being polite.