i've always found it to be way harder to run on a treadmill than outside, mostly just due to boredom.. 6 miles at 7 minute pace on a treadmill is really mentally taxing for me, and it feels "harder" than going out and running 8 at 6:15 pace outside.. part of it is just mental, but running is a high % mental anyways, and if your head isn't in it, it makes it a lot harder
i also have trouble running fast (sub 5:45 pace or so) on a treadmill because i'm constantly worrying i'm going to hit the front of it with my foot, or step off the side, so i'm always looking down and trying to make sure my stride is alright, which also makes it tougher