I might have pics. (don't believe me without pics?) Yield diff was about 5% not enough to say FF/promix suck, but I think enough to say they are over rated (since they were behind the MG)
I have nothing against FF. I do think its a fine product. (but paying for shipping of it??) Just silly. Don't we all keep hearing here "Its a weed after all" I don't fertilize my crabgrass, it grows just fine on its own. I love how its "just a weed" when it suites ones argument, then when flustered, it becomes a ultra delicate plant. MG is good for tomatoes, and pot is as close to needs to a tomato plant as it gets. I also don't understand why MG is great for every other plant *but* pot. Ain't it just a weed?
Most newbies grab MG before any research. They add nutes to it and guess what? It burns the plant. Then blame MG because *they* double fertilized and burned the plant.
As for disadvantages of MG mc soil... it has nutes in it. Makes it tough to fix a prob. but Natures Nectar has a line of single nutes. Just N, Just P and Just K so if a def props up, you can adjust (don't see why you would need it) If the plant is burning, check the availability of nutes per pH, and adjust the pH *off* to block the offending nute.
(I use some epsom salt in flowering. One could add CalMag if they want. But I think that per my strain that needed the extra Mg)
Also if its stored outside at the store, it *can* have bugs in it. Try to buy when they just get it in. Also keeping the bag inside the house will help too.
As for pros: Just add water pH'd of course.
Starting seeds in MG mc might be a bit much (I have a ton of left over promix, so I'll use that if I grow again) You can pick out the prills. Not much work for a seedling cup/pot
Also if you are so concerned about the nutes, buy a bag dump in a pot and water it for 3 months. Nutes all gone. (can't flush, the prills don't wash out like soluble salts of 'normal' nutes.)
Define huge. Tall isn't the net result. Short and bushy is better. More to the plant's appearance then the soil and nutes. Lights, ventilation, temps, humidity, and most of all genetics, (and major strain: sativa or indica dominant) Tight node spacing is what I would look for most.
Going by those pics, I gotta go after lack of light. Look rather well fed too. Skinny stalks are just like or muscles. no work, they stay small. A fan blowing on the plant so it *gently* rustles strengthens the stalk. Plants shouldn't need a stake. Maybe when you get those heavy buds. (or with LST or other non-natural methods)