A lot of characters have ended up this way. Moving from complex, interesting, characters to more one dimensional cutouts. The fact of the matter is it is easier to write characters this way. When trying to juggle as many variables as this game has flanderizing the characters is an easy corner to cut to keep them consistent across all the other variables.
Robert has actually been the most consistent in all honesty, especially in comparison to Ivy, Louise, Stan, and Allison. Even in the early development of ORS he was at best described as a ok one night stand type of fuck by Lena. Lena has also consistently held the belief that he's not particularly improved at giving her pleasure either. He's always been portrayed as a somewhat pathetic character who thinks he's more important than he is. He is slightly above average in many things; looks, self defense, and income; but it's also been clear that he is a charisma vacuum, not particularly intelligent, and that he doesn't have much in the way of mobility.
Compare that to Ivy. She used to be the slut with the heart of gold. Tried to do right by Lena and Holly, but has not become a manipulator and dragging those girls down. Allison's loving path as you said used to be interesting, but then became a repetitive: complains about work > fuck me > ignores clear signs of affection > laments that they're not a couple. I think the foundation of a good genuine romance is there, but there probably should have been more of a progression in their relationship by now. Basically where Cindy and Ian are is where Allison and Ian should also be, probably even more so, since Allison doesn't have to worry about breaking up with someone else.
