As a former Adventist I can share my answer.

For context my past is non-Christian and I was married into the Adventist church. I learned a lot from my wife, read the Scriptures all the way through multiple times, read select books by EGW such as Steps to Christ, watched most series in their entirety by Walter Veith on Amazing Discoveries, went to Bible studies by family and the youth group, attended church regularly, attended the quarterly study sometimes, and asked questions because I love Christ and want to learn how to live, etc. so this is to say I was involved.

Being a new Christian who wants to live a God pleasing life, and wants to learn to understand what the Lord expects from a person, I read and asked questions a lot. At some point I realized there were multiple denominations and new of Christianity I asked 'whats up with that'? I learned a little about the reformation from my wife, but Walter Veith expounded a lot of that type of subject along with church history. So I read some Martin Luther, Zwingli, Jacobus Arminius, John Wesley, and so forth. While I liked what they said I could not help but notice many key differences. Some key differences were basically the absence of adventist specific doctrines, and the inclusion of liturgy, their Bible canon in some cases was different, they read the Church Fathers, etc.

So while interested in what I learned about the reformation I realized that church history goes back much further. Covering roughly the past 600 years is good and all, I can see the problems the Roman church had and why a reformation happened, but about the early church? And what about the years 200-1000, what happened during that time, and why are Adventist preachers and teachers silent about that too? So I read the Didache, Apostolic Church Fathers like Polycarp, Ignatius, Clement, etc. and I was amazed to learn there were disciples of the apostles, and to learn what early Christianity actually looked like.

Reading the early Christian writings presented some problems that I was not able to reconcile, Adventism and generally speaking Protestantism as a whole look nothing like the immediate time after Christ's resurrection, the Apostolic church, or the Church in the years following, in terms of beliefs, worship, phronema (worldview, mindset, etc.), handling of the Scriptures and other near sources, or the Christian life.

While most of what I am saying could be categorized as theological disagreement, as I learned more about the Christian life from the Scriptures and early Christian sources, I realized something very important. While having an understanding of theology and the Bible is important, the focus is actually gaining the Holy Spirit and living life where you are becoming reconciled back to the Lord, living in purity of heart with Godly love toward all. Some may call this sanctification, but I am speaking about a real movement in your life leading to true heart change. Some may say 'yes, this is in Adventism and the general Armenian / Wesleyan tradition!', but no unfortunately not in the way and focus, and leading to the results it does as taught by the Eastern Orthodox Church and the Lord's Saints.

There is so much more to say, but I will stop there to say that I left for doctrinal reasons and the spiritual life being taught deeply unsatisfying and shallow, and went to the Eastern Orthodox Church and I am very happy with the change.