I've had many discussions under the title of "Christ in the Old Testament." But perhaps the issues would be seen more clearly if we labelled the debate: "God in the Old Testament."
And actually, the fact that those two titles sound quite different tells you everything you need to know about the dire Christlessness of evangelical God-talk.
We (and I include myself here in my knee-jerk western deism) imagine that there's a bed-rock deity called "God" who is obviously the God spoken of in Genesis. And then we discuss whether the Patriarchs also knew this shadowy figure called Messiah. And we debate how Messianic certain discrete verses are, and to what degree the author was aware, and to what degree the first audience was cognisant of specific promises and appearances, etc, etc. But we almost never challenge that view of "God" which we all signed off on in the beginning!
Thus from the outset God is defined as - essentially - 'the God of monotheism' (broadly conceived) and Christ is defined as a nuance to a more foundational divine reality. Then we spend all our time debating how clear the nuance was!
But what if, from the beginning, Elohim was not the god of Aristotle! It's a shocking thought I know, but let's run with it. What if He makes all things by His Spirit and Word and says "Let us"? And what if this is not something that needs to be kept in check by a hermeneutic that expects only the omnibeing? And what if the LORD God stoops down and breathes into Adam's nostrils and what if, under the name "Voice of the LORD", He walks in the garden in the cool of the day and encounters the couple as a divine Person.
How much clearer Adam saw God than us! Without the "benefit" of our western theistic presuppositions, he sees the "very God from very God." He doesn't think in that exact language, but he certainly doesn't think in unitarian categories either. To think of "the Son" as something extra to his simple belief in "God" betrays disturbing assumptions about who we think "God" is.
Who is this "God" for whom the Son is an addendum? Why on earth are we beginning the Scriptures with that "God"? And if the primary truths about God are unitarian, is our own faith primarily unitarian, just with a Jesus nuance?
The question is deeper than "Christ in the Old Testament." It's deeper even than "God in the Old Testament." It's the question of God. Which explains why the issue can get quite heated at times. But also why it's so crucial.