Treadmill speeds are not equivalent to outdoor speeds, for a few reasons.
1) They're rarely calibrated correctly
2) Even if they are perfectly calibrated, the pace displayed is the average pace. When your foot is on the belt, however, your mass slows it down. The motor controller increases the power to counteract the extra resistance, but it always lags. As a result, the belt is typically moving fastest when you're airborne and slowest while you're planted.
3) Air resistance at sub-6 pace is significant.
The 1% incline rule of thumb is actually pretty good way of addressing these issues, but it obviously depends on the equipment.
Things that can make the treadmill harder:
1) Heat. Without air movement, you can really start cooking after 30 minutes at a moderate effort. Air conditioning and a fan are pretty good at addressing this.
2) Lack of familiarity. If you don't regularly run on the treadmill, you may actually end up working harder because you'll be tense and focused on staying centered on the belt. This is pretty regularly observed with VO2max measurements. A good lab will tell you that you need to practice running on the treadmill before getting tested, otherwise your numbers won't be reliable.
There's also one thing that can cut either way: cushioned decks. Pretty much every non-research-grade treadmill will be very, very soft. The springiness of the deck can be a little propulsive, sort of like running on a faster indoor track. That said, some decks can be so soft that they're unstable at higher speeds, which can ultimately cost the runner a lot of energy. (Woodway treadmills, notwithstanding their incredible reputation and high quality, are extremely soft, and I would not recommend doing workouts on them.)
I still think the treadmill is a fantastic tool, but ideally you'll use the same equipment every time. That allows you to dial in your equivalent efforts. On most health club treadmills, I need to be going about 20 seconds/mile faster than I would be outside if I want to get my heart rate to the same level.