I wouldn't say that a CDL Mill is always the wrong choice, nor that attending a CDL Mill ensures failure.  Many trainees will go on to a successful career but many will not make it through their first year over the road.  Poor training is a strong contributor to this problem.

  There are, however, limited instances in which it might actually work, but these are unusual and there is usually a better option. 

Drivers who have been out of the industry for a few years often return to find that their experience is, in the eyes of the carrier, "outdated". In the case of an "outdated" driver, a short program is likely the best option. Many reputable schools offer refresher courses for this reason.  They would likely be a preferable alternative.