Authentic lineages is of paramount importance for Tantric traditions to have power. Otherwise, the sadhanas wouldn't bear fruit and the no attainments can arise. However, there are other factors involved as well such as having clean samaya and the person receiving these teachings must be a worthy receptacle. Hence, Lamas always put their students to the test, especially those that they have earmarked to carry on a certain lineage and they can only do so if they have 'passed the test'. Hence, you get stories of trials and tribulations of Milarepa and Marpa, Naropa and Tilopa.