I was in Breslov yesterday and I heard someone learning Zohar. I did not say anything to him about the question of it's validity because I think that some  parts are taken from earlier documents of mystics that were later incorporated into it. Still at the same time he was learning Zohar, I was learning the part in the major book of Rav Nahman about גם בהתקרבות להשם יש יצר הרע של התלהבות יותר מדאי (Also in coming close to God there is a evil inclination of overdoing it.--getting overly fanatic.) And that you see with people that get involved with Zohar.

[The main  issue with the Zohar is the phrase 'even though' עם כל דא which is a phrase from the middle ages. It is used all over the Zohar. It was made by the Ibn Tibon family to replace an older form of saying 'even though' which was אף על פי or אף על גב/ So it was not written by R. Shimon ben Yochai.]

While it is true that many great sages held of the validity of the Zohar, still this historic information was not available at the time.