I would agree that the rhythm method is based on the assumption that all women have the same cycle length, and ovulate on the same day in their cycle each month. The reality is that a woman's fertility is a little more complicated than that.
The rhythm method is more effective than simply "taking chances," but reproductive science has come along way in helping women identify their specific ovulation days each month.
Newer Natural Family Planning, or NFP methods such as the Billings method, the sympto-thermal method and the ovulation method are over 98% effective. These methods work with the women's body, not against it and is a completely moral choice when a couple has reason to avoid pregnancy. You can learn about these methods at www.nfpandmore.org. They offer a free book that will teach you everything you need to know.
I also have written several hubs on the subject if you want an easy way to get a little more information.