You can totally use NFP successfully and there some great resources available. Like any method, it comes down to your willingness to give it a fair shot. I have used NFP for 3+ years to postpone pregnancy, and have been very pleased. To clarify something posted earlier, there is no contraception use with NFP; you abstain during the fertile times. FAM does promote contraception use during the fertile times.
Cycles vary for a number of normal reasons, and they may change over time, for example, cycles naturally become longer during lactation and perimenopause. If you’re curious about improving your cycle lengths, check out Marilyn Shannon’s book on nutrition. It is sooo insightful. It’s called Fertility, Nutrition, and your Cycles or something. It has a bright pink cover, you can’t miss it.
You asked about restrictions on NFP. Well, one of the beautiful things about Natural Family Planning is that is natural, it’s 100% based on your, personal fertility signs and backed up with solid scientific data, so you rest assured about the method’s efficacy. It’s not harmful and it is immediately reversible. There are some posts on this fertility board that explain the method in more detail...check them out and feel free to email me if you have any questions.
Art of NFP
Take Charge of Your Fertility
And a new book I read about on Amazon, can’t remember the name