I've never had a problem with the 12-4-8 MG nutrients, (which I use in emergencies) and no...there's no evidence it lowers quality or effects. Some folks try to make a name for themselves by slamming crap they know nothing about, and parroting others misconceptions. They are usually the ones that never back their statements with proof, and blame their tools for crappy results. The nursery that supplies my potting soil just moved out-of-town, so am going with MG or Supersoil. It's what's available, and I've worked with both before. No big deal, except the MG and Supersoil are almost double the price of what I was using.

Sneak-up on the maximum to acclimate the young plants to the nutrients. Best to start with 1/4 tsp:gal for first week or two, and watch for results. A full dose is 1/2 tsp:gallon of water per week.
Give the soil a good flush a week or ten days before switching to 12/12. (you aren't flushing the plant, you're flushing the excess nutrients, salts, additives and crap from the soil)
If your soil is pre-fertilized, you'll have to make a judgement call with when to start the nutrients, and how much to offer the plants for the first couple of applications. It's ok to underfertilize for a few times till you get used to the dilution rates, but overdosing can kill a seedling or freshly-rooted clone overnight.

I do agree with the warning to stay away from the MG 6 month soil though. You'll want more control than the 6 month mix allows.

Quite a bit of nitrogen in the flowering mix, and admittedly, I'm curious of what the results will show.