Perhaps - I'll have to try incline again now that I have have The Watch for my workouts. I stopped using incline years ago as it really didn't do much for me compared to running up a real hill, and then my trainer explained why. It made sense once I thought about it. Another factor that makes a treadmill easier than outdoor running include lack of drag from moving through the air (let alone a headwind on a blustery day).
While I do use treadmills for a number of reasons as an adjunct to my outdoor running and hiking, I keep in mind it's a little less "work" overall, and too much of it in place of outdoor running affects my adaptation for the latter.
Another exercise machine I found lacking was the "rowing" on my home Bowflex gym. While it is a satisfactory machine for strength training at home, the rowing aspect never made sense to me. It would barely raise my pulse. Then I realized the only energy consumed is in the inefficiency of my own muscles and movements. A real rower (or stationary bike) dumps your motion into some kind of device ("sink") that wastes it as heat, such as a friction belt, fan, or water chamber, never to return back to you. The Bowflex and systems like it return the energy back to you like a spring (or gravity, in the case of lifting a weight up and down).
im a bodybuilder and use cardio as a tool. If i could not do any cardio i would love it as i don't really like much cardio. I noticed this years ago. i can do normal hiit training on a treadmill running at 10-11mph for 20 seconds will raise my heart rate to 150(after working out).
If i have the treadmill on an incline of 6-8 i only need to do 6mph to get it up to 150(after 20 seconds) and it stays there longer bringing it back down to my normal 3.8 walking speed when i do intervals.
I wouldn't say one is easier than another running outside takes a toll on my knees. depends on the level you are doing on a treadmill and outside running to really figure one as being "easier".