I couldn't figure out what to search for , so I thought I would just ask.
I Would like to know if anyone has messed anything up with using tuners , turbos and other aftermarket products. Don't have to say what name of product, just what it was and what you did to cause the issue.
I am looking to do some mods, and would appreciate any heads up on what NOT to do.
Thanks in advance on any input.

Yes, problems have happened. The usual damage is to the Allison when adding power. If you stick with a mild 60-70Hp you'll be fine, if you are planning to change the turbo or do some of the more extreme things just do your research. Pretty much anything that the average joe will want to do to his truck has already been done, and it's pretty easy to find out how to properly add lots (300+HP) of power reliably.