1. Assumption: NFP doesn't work - it's one quick trip to pregnancy.
Truth: All modern NFP methods - Billings, Creighton, and SymptoThermal -are highly effective. Like 98+% when used by highly motivated couples effective. In one German study, the sympto-thermal method was 99.4% effective for couples avoiding pregnancy.
2. Assumption: Only crazy religious Catholics use NFP.
Truth: Many NFP resources are Catholic because of Church teachings encouraging it's use, but NFP practitioners come from many religions. In fact, "Taking Charge of Your Fertility" is an entirely secular guide to charting, and many women use FAM for purely secular reasons. (TL/DR: Crazy health/eco conscious couples use NFP/FAM too!)
3. Assumption: If you don't use artificial birth control, you will have 20 kids just like the Duggers.
Truth: You probably wouldn't have 20 kids even if you refused birth control AND NFP. Breastfeeding and the natural decline of fertility cap most families long before 20 kids. With NFP in the mix, you can certainly limit your family size to what you feel you can handle.
4. Assumption: NFP is easily learned from a book.
Truth: I'd recommend learning in person - books are great but the confidence from in-person teaching is impossible to replace. I'd also warn against learning NFP from some of the self-taught individuals online (cough cough reddit). Make sure you have reputable information and if it sounds weird double check.
5. Assumption: NFP doesn't work for women who don't have textbook cycles.
Truth: It can take a little more time to learn when your normal isn't textbook normal, but NFP is especially worth learning if your cycles are wonky. Charting will give you a much better idea of what is going on and if there are underlying health issues. It's also still effective at preventing pregnancy - none of the modern NFP methods are based on a hypothetical woman, they are biased on the unique cycles of each individual woman.