I do not think there are any CONS...but I am not the man when it comes to cars. I am very careful and do overkill research before doing anything to my car. There are several things that come into play with meth injection. The best I found that explains meth injection is this video:
If not I would be typing forever. I spoke to 3 friends who have 1 series and have meth, two of them for at least 3-4 years and no damage at all. I am going to purchase my meth here soon.