I am not a great fan of exit interviews. The big problem with exit interviews is that people often don't know why they are leaving.
In my basic model of human behavior people leave institutions when they no longer feel comfortable in that particular institution. However people often do not understand their own reasons for feeling uncomfortable...they may not be able to explicate it.
When I was doing survey research in the early 1970s I was asked by several churches, who were losing members rapidly, to do survey research for them. Research to find out the reasons people were leaving.
There was very little I could do to help because, as I explained to the church people, a church member may feel that there are other things to do more interesting than going to church. To measure what other things are more appealing is difficult. The church can do nothing about competing with sports or sailing or whatever it is that the former parishioner is now doing.
With the perspective of history to look back we can now see what was happening that I could not see at the time. People were leaving the liberal churches in America for one reason: the churches had become politically active in liberal causes. That was simply not the reason ordinary Americans were going to church. Parishioners certainly did not support the very liberal positions that their churches were taking. (That is still true of the Presbyterian churches anti-Jew publc stands.)
We know from looking at the data, after the fact, that these very same people who had been going to a Methodist Church were now going to Evangelical or charismatic churches that were non-political and focused on family life.
Exit interviews would not have helped the churches who came to me. Exit interviews do not tell you what people are going to do or why.
I do not apply this same logic to exit interviews after people leave the voting booth. Those exit interviews usually suffer from a different problem. They suffer from bad sampling procedure and unwillingness of voters to tell anyone who they voted for.