MG soil is great for tomatoes and other garden foods. But mj is pretty much a weed, and doesn't require so much nutrient as is usually in MG soil. As some others have stated, overwatering is the biggest problem when growing cannabis in MG soil, releasing too much nutes/ferts at once, causing burning and stunting. I've also read many reports of MG soil being infected with fungus gnats (bad news). As it is so much safer, especially for a beginner to grow in cheap potting soil which has no nutes added, and just start adding light nutes/ferts after 3-5 weeks from seed, many experienced growers recommend that.
In spite of all the written information available on the internet, a novice grower is going to have to play a little by ear with the first plants they grow.
I have seen a few folks do some good grows in mg soil. But just to be on the safer side, I would recommend cheap, plain Hyponex potting soil (walmart), plain perlite, SM90 used per directions for pest prevention, and floranova nutes. With plain soil and plain perlite, the plants can be more forgiving of overwatering. Works well for me, and my thumb ain't exactly green...
Good luck.