It is my opinion that we should not try to keep God in a box. I have never been "slain in the spirit" but that does not mean that God can not use that to bless those who have been.
God does not have to always have a yellow jacket sting your ear to heal an ear infection like he did for me.. (long story)
My point is that God is awesome and He can use what ever he wants to bless His children. We will be judged by the standard we judge others. I would advise giving lots of grace and mercy and not dogmatic condemnation of other's experiences.
Let's all become fans of mysticism while we're at it. Where do we draw the line? Mysticism is at the very core of many Christian's lives. It isn't new. It was even that way in the days of the New Testament.
No, God is not placed in a box. However, if it is not in Scripture, I am not about to say the experience was from God. Scripture is my only litmus test for Truth. Nothing else is an acceptable substitute. Because there is zero foundation for being slain in the spirit, I cannot, in good conscience and faith, allow myself to entertain the notion.