Multiple measures provide more certainty - 1-2% chance of pregnancy with NFP X 2% chance with barrier methods = 0.04% chance of pregnancy - same principle holds true with any set of independent safeguards for anything. Sterilization would provide even more certainty - and I am not sure why when pregnancy itself is a threat to health and life, it may not be surgically prevented - when licit treatments for other conditions, e.g. fibroids, do sterilize. Fibroids are rarely life-threatening.
John Paul says in Love and Responsibility that abstaining during the fertile time is difficult for the woman - her desire is strongest then. He is right. It is even possible that the woman will experience NO desire when she isn't fertile. Using NFP to not conceive means that the woman can not offer her husband the gift of her strongest desire - or sometimes any desire at all. In these circumstances, her pleasure and fulfillment are likely much diminished.
Some couples experience difficulty and resentment when they have to wait two weeks plus. A very devout nurse, of about forty, told me that she and her husband were now doing NFP but they could not have managed when they were younger.
Saint Paul said it is better to marry than to burn. Sometimes we are called to be married and to burn - as when spouses are apart from each other. To do so when they are together - some might call that perverse.
Mrs VS