Examples of bad tuners are those where you send them the PCM and instead of custom tuning swap and mail you back a PCM that came out of another car thus IDs may not match or use the same calibration and do cookie cutter tuning where they may or may not even change the VIN ID and flash that into your PCM.

If you want a correct and valid tune make sure you determine if in fact the tuner knows what the hell they are doing and spend the effort to properly tune your PCM.

Generally find those tuners that charge a cheap price put no effort and not giving you infact a custom tune or a shop that claim they are tuning are in fact using another source for tunes and are doing it in total blind of the makeup of the car