its a 100% crock. each and every car has different PCM tuning per PCM, you can only piggyback certain systems and other certain systems can accept a "chip". adding a ROM chip can work on the older OBD 1 cars but the newer OBD 2 cars have programming to specific outputs to reduce the chance of emissions degradation. I know that the new boards(ford at least) in PCM's do not have an additional ROM slot for add on devices and has a anti-tamper module which will set the car into theft mode once programming is outside the boundary of programming, which i have been told is around 15% +/- total emissions compliance. the way to get around this is to program with a handheld module. companies like diablosport/sct/hypertech have been cracking the original codes for years and have produced some of the best modules for the money.
the rule is if its under $100 its a crock. most handheld tuners are in the $300+ range but offer much more than just a soldered FET onto the board. even the old soldered chips for the OBD 1 cars with $150+ and have now since been foreshadowed by tuning software like tweecerRT and LS1edit.
what type of vehicle? i have been modifying cars and trucks for a long time and worked for Ford Motor Company for 6+ years as a district engineer and technician. I also took courses with Kia/Hyundai on the GDS systems and suspension design and analysis.
pm me and ill help you out any way possible....