I'm a huge fan of indoor riding. Numerous benefits such as good for bad weather days, less to clean up, can be a time saver, safer at night, better for structured intervals, etc. I seem to get a great leg workout. You can just concentrate on pushing the pedals. Great for benefits in FTP. However whenever I go for several weeks without riding outside, I always suffer for a ride or two. That tells me there's something about outdoors you can't replicate inside.