The Nitrogen (N) in Miracle Grow is a type (synthetic) that is harsher than an organic N. Easier to burn a plant if you don't know what you're doing. It also is projected to leave salts in the soil that are hard to flush out if need be. Although I believe all fertilizers will leave salts or residue of some type in the soil.

As for 'everyone' saying it's no good? Depends on what web site you're on. On another site I asked about the same type of question and a lot of people came to the defense of chemical/synthetic fertilizers. But most were experienced growers that knew what they were talking about and could probably grow mj on a slab of concrete.

The 15-30-15 can be looked at as 1-2-1 also I believe. But that might be different for different types of fertilizers or areas of the country. I believe it's just showing you what percentages each ingredient has as compared to the others. And of course the 15-30-15 would be better suited for flowering or just getting started with flowering because of the higher percentage of Phosphorous (P) which is represented by the second number.

At least that's my take on it all. And I'm no expert. Just learnin'.