When you do Agile development you really need good people otherwise things won’t work. And my answer is have you ever seen a waterfall project delivering good software by putting baboons on it and giving them bananas.
This average thing is completely wrong. To produce software you anyway need good developers no matter what. You choose whatever methodology you want you are not going to deliver anything.
Scrum – Getting Started without Getting Burned
At the end of the day the best predictor of any project success is the people. Scrum can help good people work better, but you actually need the good people or you need to grow them.