The method has only ever been used for a small number of clusters. This is because for the method to work, the cluster must be quite close to Earth (within a few hundred parsecs), and also be fairly tightly bound so it can be made out on the sky.
This technique has only been used on a small number of clusters. It requires that these clusters be relatively close to Earth (within a few hundred light-years) and tightly bound, making them visible from our perspective.