Here’s what we know about Steffon Baratheon: we know about his childhood at the court of King’s Landing, his close friendship with Tywin and Aerys, that he fought in the War of Ninepenny Kings and saw his father killed by Maelys Blackfyre, that Aerys named him to the Small Council after the Defiance of Duskendale, and that Steffon’s mission to Volantis may have been a prelude to Aerys replacing Tywin with Steffon as Hand of the King.
What do we know about Cassana Estermont, by comparison?

As for the Lady Mellario, A. she’s not dead, and B. we know quite a bit more about her than other members of the Dead Ladies Club. We know about how she met Doran, we know a lot about her marriage with Doran and their conflicts over Dornish customs and child-raising practices, even to the point of threatening suicide if Arianne was fostered, and we know how she felt embittered by the terms of their separation. But it is true that we should know more about her relationship with Arianne, given that Arianne was raised by her for some time.