Depends on how much the exit interview is genuine exercise in finding reasons and how much it is simply done because that's what your supposed to do.
Then again, how effective can they be?
As suggested by HgTX the outgoing employee will not and should not be saying anything that isn't in his/her own best interests which often doesn't include telling them what's really what.
What's the point anyway? finding out something ain't right through exit interviews suggests it is too late, and especially so for that employee (like voluntary redundancies, it is always the best employees that go because (a) they are better able to see when the company is going no where and (b) they are the ones with the best shot at a new job).
If something isn't right they need better means to detect it.
So, if anyone has had an exit interview, what happened, how did it go and was there any discernible point to it?
Anyway, the best exit interviews are with your colleagues in the pub for a bevy or two on your last day. This is where you'll let your hair down a bit and the best means for the management to discover what is going on would be to get the pub landlord to provide some covert CCTV footage (with sound); no good waiting for a report from their brown-nosers, these guys wouldn't know the truth if it bit them. Of course, for the real deal they ought to have ongoing covert video surveillance of the coffee machine.
Then too, as HgTX also suggests, management will only act on things they believe. They will not believe that they are a problem and may prefer to believe that you are the problem because that way, the problem is going. It is never good news to think they have a problem with themselves or the remaining workforce.
JMW