At the end of the day, flat rate can work well if your shop has a decent mix of warranty and customer pay work. You need a repeat customer base who is willing to buy a high percentage of the recommended maintenance and repair items. Certain brands and income brackets will greatly influence this.
Except alldata and other labor guides have been dropping their customer pay labor times to be barely above warranty.Warranty work is not currently as bad as in the old days for many Techs.
Some states have mandated better flat rate pay for warranty work. Example; In Illinois the techs currently receive the specified repair times multiplied 1.5 for warranty, and recalls.