Physiologically, I have no idea if it’s easier to make power indoor or out, but I’ve never felt there was much of a difference. It’s hard to tell, though, because I mostly do structured rides indoor but never do them outdoor, so I don’t have apples-to-apples data. Looking at “JRA” (just riding around) on the trainer compared to JRA outside, that’s tricky to compare as well, for two main reasons: 1) I usually group ride outside, and 2) being indoor allows me to ride harder over shorter periods without fear of being dropped or running the tank to empty. I think the variability in my ride data, in terms of power peaks and averages reflects that, with higher one hour averages, generally, on the trainer, higher shorrt period peaks on the road.
Further complicating comparison for me is that I’ve been riding a PowerTap stationary bike for ~13 years at the studio, and have had a couple of different turbos at home, including the Kickr I’ve got now and have been using since ‘17, I think. Those two are extremely different— the Powertap bikes use dial adjust friction pads on heavy freewheels— and engender different types of indoor rides, particularly notably for resistance changes in Zwift with Kickr. They’re both hub power meters, and outdoor I use pedal power, which should read a little higher accounting for drivetrain losses; I’ve had Powertap wheels and run my Assioma pedals on the Kickr, and don’t see a lot of difference there either.
It’s just different kinds of riding, each with diverse goals and opportunities, so I find it hard to compare, even though, broadly, both power and PE align for me on road and trainer. When I’m fresh and good, what feels like 150w, 250w, or 400w at the legs usually is on the bike and trainer both. When I’m tired, what usually feels like 400w is actually 150w on both bike and trainer, too! 😂