Actually, depending on how you look at it, Christopher's condition as having autism makes him probably one of the most truthful narrators you could possibly imagine. He himself says that he cannot tell lies and has to tell the truth. However, what your question is pointing towards is the way in which we tell the truth from our own perspective. And this is the important point for this excellent novel. Christopher does, as he says, tell the truth, but we are forced to recognise that he is only able to tell the truth from his reality and understanding. As his autism makes his understanding of human interactions limited, we are often in a position that we, as readers, are able to see the real truth of what Christopher is observing, when he is only puzzled by what is happening. You could use some of the interactions between Christopher's father and his mother, or his father and the neighbour, Mrs. Shears to see this. Christopher, throughout the novel, struggles to understand the complex actions and motives of the adults that surround him, and why they have deceived him, such as his father lying about his mother's "death."
Thus I would question the words "unreliable" and "ineffective" in your statement. Christopher is actually incredibly truthful, but it is his ability to understand the complex interactions of humans that make his account partial and limited.