I wouldn't assume that every companion will have an unique ability. Astarion and Karlach have them because of unique backgrounds. But not every companion necessarily has to have some unique ability.
Indeed, I think you are right on this. And it also makes sense because Larian said they were trying not to make people playing customised characters feel out of place, as some people did in DOS2(I don't really have a source for their note, it's just a vague memory, so correct me if I'm wrong).
In this sense, it is reasonable to avoid going to hard on making each companion "mechanically special", if I'm expressing the idea right.
