I wonder why you ask. You state that your fitness has improved a great deal - res ipsa loquitur.
I am a track racer and I workout at least twice per week on rollers at my coaches' cycling gym. And there are several roadies that do as well. My coach is there with me when I am on the rollers calling out numbers and encouraging me to go harder. I could not / would not go as hard or as long if I were alone on the road or track.
IMO, riding the rollers develops better power development from the core with less input from the hands. I have been doing max effort intervals on the TT bike on the rollers and it has helped dramatically in controlling the pursuit bike on the track holding the black line.
IMO, riding on the rollers, independent of weather, is a competitive advantage to those who know how to use it.
Whatever you can do, or dream you can, begin it. Boldness has genius, power and magic in it. - Johann Wolfgang von Goethe, writer