With strict boundaries for the relatives a partner can feel safe in the relationship. They do not worry about where they stand with their spouse, they know they come first.
A MEM does not set boundaries with the family of origin but rather with the spouse. They will not tolerate a spouse complaining that they were treated unfairly and often cruelly and rude by the MEMs family. He will find fault with his spouse rather than acknowledging that his family of origin is dysfunctional and destructive. I do believe “good fences make good neighbors” as Robert Frost writes in his poem as much as good boundaries make good relationships.
If you are dating and your partner and the family have no boundaries, you will be much happier leaving before you are committed and possibly have children in the mess.