I guess that depends on what you mean by efficacious.
The intercession of the Saints is no more a spiritual vending machine than is our own prayer to God. We ask in ignorance; God answers in love and wisdom.
The Church gives us the Saints not only for the power of their intercession, but also (perhaps even moreso) for the power of their lives’ witness! As time goes by, I find that knowledge of the lives of the Saints becomes more important to me. When my heart is aching from loneliness (I’m a divorced single mom), I remember the witness of St. Rita. When I feel overwhelmed by the needs of my students, I am encouraged by the witness of St. John Bosco. When I am tempted by anything that would separate me from Christ, I turn to the witness of Saints like St Clare, St Francis, St Teresa of Avila. When I feel surrounded by darkness, St Teresa of the Andes gives me her example.
I ask these (and many other!) Saints to pray for me. But as I get older and, for now, life gets harder to bear, it is the lives of the Saints that is most efficacious for me.
So as you get to know the Saints, do ask them to pray for you AND get to know how they lived their love of God.
God bless you!
Gertie