I've heard some Christians claim it counts as "witchcraft" (KJV translation of φαρμακεία (pharmakeia), which is specifically referring to drug use, and the origin of the English word "pharmacy"). Considering also the use of the word "entheogen" (meaning a substance that manifests a god within you) by some users of psychedelics, and historical pagan use of psychedelics, this seems reasonable.
Sure there is. Churches would love to have a monopoly on providing religious experiences. Any profound experience not associated with church practice is a direct threat to church hegemony.