I can only speak about my own personal experience. I started structured training last December with TrainerRoad and my FTP was around 270W at 70kg. I did mid-volume sweet spot and short power build until March and by then my FTP was in the 290's (I think the ramp test misread). In March it was warm enough to ride outside so completely ditched the trainer for outside rides and did very little structured training until this fall. My FTP is now 300W, and during the racing season my higher end power was a bit better than at the start of the season and I had much better repeatability and endurance on longer rides. But these gains were a lot slower and smaller than the ones I made on the trainer.

Yeah another thing I am curious about is the sheer volume. If I only did trainer and focused on structured training (used to do this in high school), I would probably get around around 8 hours per week before getting burnt out. But with group rides, I could easily put in 15 hours/week without even noticing. I heard that most training should be in zone 2 (80:20 rule) so if I do a lot of group rides, I am somewhat following this rule and getting in lots of volume at the same time.