All that online data is fine and maybe in general terms certain chains, cogs, and their wear applies to loosing watts. However, until you start testing your own drivetrains and your trainer none of the generalities matter. I would have thought my wheel-on Kickr Snap versus my Quarq XX1 that measures at the spider would be way off. I mean my Kickr Snap has to deal with the chain, cassette, tire, tire pressure and drum. While my Quarq avoids all of that. So in theory I should be losing watts to the drivetrain and wheel, but after 5 workouts at all different power levels and cadences that’s just not the case. Yes there are deviations at times depending on the workout, but the avg power and NP are always within a couple watts of each other.
