That it is a lovely custom that too many use superstitiously rather than prayerfully. Catholic prayer is physical, as well as vocal and mental. So there is nothing wrong with engaging in certain actions along with prayer. But if the actions are considered to magically bring about the result desired, then that is superstitious.
In other words, if you bury a statue of St. Joseph as part of a novena or other prayer devotion to him for his help in selling your house and then later give the statue a place of honor in your new home in thanksgiving for St. Joseph’s intercession, there is nothing wrong with that. But if you decide that burying a statue of St. Joseph will, in itself, cause your house to be sold more quickly, then that is superstitious and should be avoided.