The focus of 6 month smiles is the smile and not the bite, root angles or goals that take 2-5 years to complete. It is an alternative to comprehensive ortho or Invisalign ONLY when the patient is not concerned with anything other than the appearance to the front teeth. Teeth don't move faster than normal, the case is simply done 'early'. An orthodontist would feel compelled to strive for "perfect" when many patients simply have different goals.
The brackets go back to the first molar, just like regular braces, and some posterior tooth movement DOES happen. While it is called 6 month braces, some cases take 3-4 months and some closer to 9. 6 months is simply the average and is easier to market.
6 month braces is NOT comprehensive ortho, and "perfect" is not the goal. If you have an overbite, you will still have one this way. If you don't want crooked teeth, this is a great alternative.
Retention of cases after braces come off is determined by wearing retainers, no matter what method is used.