This is my second year of SFG. There are a few plants that I think might have done better in a regular garden, but for the most part, everything grows great.
The 6" is no problem. My tomatoes grow to 6' tall and are laden with fruit. My cukes are short-ish but growing great cukes. My green beans are also short, but REALLY productive. Herbs are fantastic, I can't keep up with my thyme. Onions and garlic -- I've got a patch of onions in their second year (letting them go to seed) that grew 5 feet tall!
This year I'm growing melons. I didn't bother trying to make it vertical, I'm just letting them spread out. One plant is taking up 3/4 of a 4'x4' box (the other quarter is the runaway thyme lol).
I reinvigorated the soil mix with fresh compost and it's rich and healthy. No blossom end rot or signs of deficiencies.
I'm growing potatoes and carrots in a 10" box, but everything else is fine in the 6". I grew carrots in 6" last year too. They just came out L-shaped if they got big enough.
I had no problem getting coarse vermiculite. Our local garden centre carries it in the huge bags. Yeah it's a little pricey but it goes a pretty long way. And the asbestos is an old issue that's no longer applicable.
Peat is a concert... but we're using Canadian peat, which I *think* is less of a problem than other areas which have been over-harvested. But again, a little goes a long way, and with both the peat and vermiculite it's only the first year that you need it.
The potatoes are in 100% compost, and I think that ended up being even more expensive than the mix!
All that being said, I totally understand those who prefer not to use the mix and I'm certainly not 100% blindly loyal to it or anything. I just wanted to give it an honest try, and found no serious problems. The 6" is definitely not a problem either.
And I would also agree that SFG, as a strict method, would not be as practical for a larger garden area. I have 6 4'x4' boxes and one 6'x4' 10". I probably wouldn't do much more than that. Any more than that, I'd rent a tiller, dig up the lawn, and add tons and tons of compost.
EDIT: And in my SFG's second year, there are now definitely worms in the boxes!