Writing is difficult. It takes time and effort. My feeling is that LArian has devoted more resources to some companions than others. As a result, some dialogs aren't as fleshed out and work as well as others. I might be wrong on this, but just the amount of scenes, interactions and quests, says that Shadow Heart and Laezel have more content and work put in them. If the same amount were put in the others, we would probably start to see more and better options to interact with Astarion and both paths would work better.
Astarion has the most content of any companion in the game, quantity wise. Doesn’t he have like six romance scenes? Shadowheart has like two.
