Everyone in Act 3 practically goes silent, most romances drop off the face of the planet.
As far as I am aware there is dialogue between companions for all romances (there are some vids on YouTube which have much of all the voice lines available in party banter) but it could be potentially bugged much like a lot of other stuff. I am not sure about cutscene specific stuff, though.
However, it has indeed been proven that some companions have a lot more content to do with their romances than others, and you only need to look at the disparity of how Minthara or Halsin has been treated compared with,say, Astarion or Shadowheart. And I don’t care for the origin excuse at all because Wyll has been equally shafted.
