The terminology “nature and strength” is used in other family law legislation. For example, it's used in the B.C. Family Law Act, and no concerns have been raised with respect to the use of that terminology in that legislation.
The term “quality” is much broader than the concept of “nature”, which has you look at different aspects of the relationship, and “strength”, which asks about whether there is a close relationship, a good relationship—those types of issues.