Mechanically, they are similar, with the main difference being that the M8 has a new console. It has a slightly more excellent display and works with Max's app-based coaching platform.
The main reason is the latest coaching platform could not directly interact with the M7, so Bowflex updated the M7 with the new console. The M8 has fewer preset workouts outside of the app. However, the M7 has 11 apps, and the M8 has 7.