I think you need both, structured hard (solo intervals) and unstructured hard (group) rides feel different to me. I prefer doing mostly solo intervals, or intervals with 1 or 2 team mates, seems easier to focus on specificity, and only do group rides as race substitutes (like this weekend coming weekend, where I wont be racing), or when I'm burnt out on doing the solo intervals and want to mix it up with some others.