As of today it is possible to plan longer routes as well! From now on, the new “Routes XL” servers were taken in use, which should eliminate the need for having to add a stopover halfway your route.
Waze was originally conceived as an app for calculating commuting routes. This meant that the algorithm used didn’t need to scale to 700 kilometers or more. While Waze’s focus is still quite strong on the daily commute of people, it makes sense to support other driving behaviour as well. Please do note that this system is still being worked on. There’s no guarantee long distance routes will always work out fine, though the experience should be a lot better than in the past.