There are several problems with "natural law". There are multiple definitions of the term. One is "the law written in human hearts" (see Aquinas' Summa Theologica, I-II, q 94, art 6.) Another is "what is found in nature". (There is a third, having to do with legal theories.)

In practice, "natural law" can be misused in several ways:

It can mean nothing more than "this is what is written in my heart", which is the same as "in my opinion". Personal opinion is not a good basis for moral or dogmatic teachings.

It can be invoked when the person does not have an actual argument. A good example is in Humanae Vitae 4, which says essentially "we know the natural law, and you will just have to take our word for it". That's not an argument, that's blather.

When it does not actually describe how nature works. Humanae Vitae says that in nature, sexual intercourse is used for procreation. In fact, many of the great apes use it for pair bonding. So the author of HV got it wrong.