I heard that apparently MG isn't formulated for plants that harvest fruit and therefore plants that will never be used for human consumption, so it contains chemicals that are potentially dangerous if ingested and maybe carsinogenic if such plants are smoked?
Whereas other chemicals that are specially formulated for fruit-bearing plants will be less harmful to your body as they are designed to promote growth of fruit that will be consumed. If you look on the labels of these they will show pictures of fruit, etc but MG doesn't.
Not sure though, I'm not a scientist! But I'd just be careful, I won't use MG now as I'm scared of this.