I can't claim to be an expert of what the soil contains, I add pearlite, lyme, azomite, myco, and oatmeal, and feed with 1/3 strength botanicare nutrients, I don't get 'burned' seedlings, and I am pushing close to 2 lb per 1000w bulb (hortilux), which my research leads me to believe is on the high end yield wise, so, when people ask me what to do soil-wise, that's what I recommend.
i like roots, but I like it because I am familiar with it, I know it's ph is low, I know what it's short of nutrient wise, etc. IDK exactly what you are trying to figure out, but I do not think that there is any PERFECT soil mixture, you just have to be familiar with the different soil mixes, and know how to supplement them appropriately.
I also do not think that is it accurate to say 'this brand burned up my seedlings' unless you're using miracle grow soil mix, or some other similar cheap stuff, if you look at teh nutrient profile of any major brand soil mix, they're pretty neutral for teh most part, what a lot of people blame on the soil burning their seedling up is usually soil packed too tightly, and not enough pearlite added to loosen it up..
I'd say use what you can get inexpensively, but ph it, and supplement with teh common supplements, myco, azo, lyme.