The difference is often your bank. Some of my banks don't even have a way to initiate the charge back process (you have to write a free form email), presumably to reduce overhead and to avoid losing cases needlessly, while the others make it very easy with a web form.

Anecdotally, I've never had one fail. I've probably done maybe 4 or 5 in 10 years. Though I despise Chase and don't use them, so that may be the key. Amex and Discover are top notch in that department.