When we say their Eucharist is invalid, we’re merely asserting that it lacks the assurance that grace is conveyed in the way it was intended by Christ. Because it is invalid, we cannot say that grace is contained and has its effect through the administration of the sacrament. However, the signatory function of the Bread and Wine is not dependent upon priestly consecration but derives its function primarily through the Word. As such, one need not argue over the validity of signs. It is in regard to these signs that those outside of Apostolic Succession maintain some kind of proper intent. In what they put forth and offer, they truly administer signs of the Body and Blood of Christ Jesus, even if the real presence of Christ is not contained therein.
Source: Apostolic Succession and Sacramental Grace