Anyone know of Fethullah Gulen and his beliefs?

Someone mentioned to me that he is a perennialist and that this is a bad thing. (Said Nursi was also mentioned)

However online, I cannot seem to find anything on this and he seems rather mainstream.

So the questions I have is:

1. What is perennialism?
2. Why is this bad?
3. What are the views of Fethullah Gulen and are they mainstream?
4. What of Said Nursi?
5. Is there anything else that would be interesting to know?

Thanks.