Reason: I was recently tredgeing through my third Dawnguard run-through (currently beginning the forgotten vale) and a thought came to me. In the companions the only one I remember seeing turn Wolf was Frakas (at least it was one of them, the exact name escapes me atm) and Aela in the Undercroft. The others we're just told they are, but we never witness them change.
With Harkon and Fam I was made to believe that they all could take VL form due to the ritual they endured, yet we only ever saw Harkon turn. Now, I could 'assume' that Serana 'could' take VL form since later on she can turn us if we didn't take the gift from Harkon...but I'm thinking that this is likely a convenient mechanic and not necessarily 'lore'.
So I put it out there to you for discussion. What do you think?
Is it safe to say that Harkon and Fam can all take on the Vampire Lord form?
Here's a followup: Could we assume that everyone within Harkon's court can all take the form?