I don't understand how somebody in good conscience can do modding services without knowing how to solder properly. In my opinion, if you use the right stuff and know how to solder properly, there should be zero complaints or rate of failure, and I've heard and experienced numerous from different sources. I won't name names, but I've fixed a number of Game Boys that friends and acquaintances have given me from one very well known modder that I am honestly appalled with.
If you do modding services, please do everybody a favor, stop it until you follow these rules. These should be understood, but unfortunately, they aren't:
Use a solder station with temp control. Don't don't don't use those shitty irons that plug right in the wall. Even some higher-end models plug right in the wall, but I can't trust them.
Stop using thick solder. This, I think, is the number one reason for sloppy soldering. Honestly, when it comes to soldering, less is more. With thinner solder, you have much more control and are less likely to fuck up.
Stop using those cone shaped or pencil tips. Just stop it. Chisel tips, please. Trust me on this one.
Stop using solid core wire. SHOULD I EVEN HAVE TO SAY THIS? I kid you not, I have received Game Boys with solid wire on the mods. If you can't guess, the wire broke off the jack almost immediately upon my inspection.
I know you might say "I use _____, ______, and blank, and I have no problems." I don't care how well the few of you do with unsatisfactory tools. If you are doing modding services for money, you are a professional. Use professional grade stuff.
Second, once you stop using crappy stuff, you might notice how incredibly your soldering has improved. The fact is though that some of these atrocities I've seen may be from people who have all of the right stuff. So, learn how to solder properly, and inspect your work. If you have the right tools and know how to solder properly, it's actually difficult to do a bad solder job. I've tried to show students in a workshop examples of soldering poorly, and it was tough for me to do it without being overly exaggerating. This means that a good amount of mods I've seen have been made by people who don't know how to solder.
A final note about some of these modders who try to defend themselves. "All of my mods work." JUST BECAUSE IT USUALLY WORKS DOESN'T MEAN IT'S PROPER!!! Your crappy job is bound to fail in due time, and you probably won't hear about it. The original poster had SIX Game Boys die on him. I admit, some of that may be inherently on the machine, but some of that is also on you.
Have some ethics and don't charge people unless you are giving them a professional job. If I have to fix one more Game Boy that looks like it was modded in a kitchen by a seven-year-old Korean assembler, I'm going to blow.
Or just keep doing what you're doing, since your incompetence is making me easy money.