Hi there, nice subject... I can tell you for sure that these "christians" can cause more harm with their prayers than anything else. First of all, through first hand experience and there is a very good book about "becareful for what you are praying for". These guys DO NOT know what they are doing...
It might not answer your question, but this books speaks also about the thin line between and the effect of prayer, which I think applies to reiki as well.. the thin line between curse and cure... If I do not send reiki with unconditional love, it can fast end as a spell??? (at least a beginner?)
AS said before, if you send reiki or a prayer with love, you will not cause harm. but are still human, so we have to be careful what we think...
Plato once said, those who pray for other should be procescuted....(those that think they know all and pray for other to go totheir church for instance..

) I (personally) think we should restart this debate, not think about if reiki is dangerous, as most of the reiki people know about the love one sends...
I really think this, as I have a evangelist parasit on my shoulders, who really intereferes with her prayers into my life...
not a straight answer to your question, but I feel I wanted to share this... a good book if you are exposed to that kind of people to give them some arguments...