I worked at a pet store while I finished up my degree and I can guarantee you that people fail for one (or both) of two reasons:
1. They get a sick or too-young animal from a bad source, like a sketchy pet store or importer.
2. They were not properly prepared at all, especially with the vital things: cage, lighting, diet, and water. Because you can always tweak and learn as you go, but if you don't have a UVB light or are not giving enough water, there's no chameleon that is going to last long under those conditions.
The people who keep both of these things in mind and prepare ahead of time and then seek out a reputable source are MUCH less likely to lose their chameleon. If you start off with a healthy individual you'll find that they are pretty adaptable, chameleons aren't made of glass, but they do need their essentials at the very least.
So don't let him scare you! Obviously either he or his customers were doing something terribly wrong because chameleons (especially panthers and veileds) usually have a great survival rate. Besides, you have all of us to hold your hand through the first weeks/months/years of cham ownership! lol Which is something they probably didn't have. So between the awesome research you're doing and the support you have from the forum, I'm sure you'd do great.