Well, I'd say one of the things that made me change my mind on NFP, was that someone I respect, a priest in France, saw no problems with it, nor did any of the French clergy.
That's a very weak reason. Lots of people support lots of priests, and priests often of approve of things they shouldn't.
As for the fact that French priests you know of approve of it - that's another very weak reason, given the national character of France.
I'm not going to say that periodic abstention is wrong or immoral, but it certainly can be done for immoral reasons. For that reason it seems to me to be a gray area of morality. The people who condemn it all outright do not have justification for their position, because you cannot prove that paying the marriage debt is wrong, or that there are not grave reasons for mutual abstention. The very terminology "NFP" is very troubling, as is the idea that young couples who seek to be married should be trained in such "techniques". (I'm not sure trads do that however)