I'm an ex-GM service tech too....they always blame problems on any type of modification or connector that isn't factory.(and understandably so,theres no license or test for anyone to work on cars)
Techs Aren't as savy as the past (low experience because of the automotive repair field and the employee turn over rate) and they train techs to shy away of "unproven" methods.(you must follow the computer or the tech 2 steps)
as in ANY part,connector,accessory,etc there a wrong and right way to use.Ive used 3m t-taps for over 10 years on very high amperage applications like remote starters,etc with no problem. the key is the correct one for the gage of wire,and stripping the insulation for 100% contact.
heres a link to 3m page about the t-tap.
i don't however like the "Scotch lock",except the largest version (brown 10ga +) because it has double contacts for each lead.
3M eStore: 3M Scotchlok T-Tap Nylon Insulated Self-Stripping Female QuickSlide Disconnect 952X, 18-14 AWG
grab a couple to test a few to see the results for yourself.you can find them at autozone,orlies,etc...they have became popular.
another point to add; use high quality (3m) male spades to plug into the t-tap.they are thicker and make a better contact/connection inside the t-tap.
The main reason was because the scotchlocs would cut must of the wire in half. Does this type cut any of the copper?