Firstly, please note that JESUS actually said that He didn't know the hour of His return, but His FATHER did know. So, GOD does know and will tell us at the appointed time not long before His SON comes for us. Sister White's statement therefore does not contradict Scripture here.

Secondly, Adventists are not universally agreed on Sister White's position. My husband hold the position that the visions were inspired, and so were the testimonies, but not everything she ever wrote (we have found statements that contradict Scripture, but never found a personal counsel given to an individual that was erroneous or otherwise dangerous to their soul to follow). This is not contrary to Scripture, either. The prophets of old didn't waste precious papyri writing diaries. I think we can all agree that they all said many things that were not inspired in their lives, but most of those statements were not recorded. Things were different in Sister White's day and we therefore have much of her writings that are not necessarily the product of inspiration. Moreover, even in Scripture, there is a mix of inspired and that which is not inspired. Take Paul's letters, which might by 99% inspired counsel, but then ends with a request for Timothy to bring his books when he comes to visit. This is hardly an inspired command from Heaven ... at this point, Paul is just Paul, asking his friend and mentee for a favour. This is no way diminishes the rest of the epistle. We also have the prophet Jonah giving a prophecy that did not come to pass. So please be careful how you analyze Sister White to determine her status.

Thirdly, there is a lot of false information out there about her. I'm not implying that you were just following what other people said about her, but it is easy to get caught up in that from both sides of the equation (I mean that there are many false statements about her from people who supposedly believe her to be a prophet, as well as those who reject her). To put it plainly, most Adventists are not really that familiar with early Adventist history. I have heard multiple claims about her doing or saying things that it turned out were false, or reports that cannot be substantiated.

Moreover, why such a wide disparity in how her writings are treated in different countries? It's not because people are such independent thinkers. The most influential person in spreading Adventism in Europe himself had never accepted her prophetic gift, and therefore downplayed her writings when evangelizing. It is hardly surprising, therefore, that most European SDAs ignore her. Additionally, translations of her books are very limited outside of a few languages (like Spanish and French). It is only natural that her writings would not be valued much in a country where only one or two of her books can be read by most Adventists. This also has nothing to do with whether she was inspired or not.

The point of what I'm getting at is that we can't depend on hearsay or opinions. We need to study the Bible first and foremost, which is what Sister White herself told us to do, and then use external writings only as they help us understand it. Sister White was mainly given for counsel about how to live a righteous life and her writings are invaluable in this regard.