There are many hidden gems among segments that are nearly impossible to discover unless you happen so see them in someone's ride or run. That is how I discovered many of my most favorite segments.
Some segments are more like virtual races or FKTs - they are long and may take an hour to a few hours to finish. You'd never see a segment like that in Segment Explorer for the following reasons:
1) Segment explorer only shows 10 most popular segments for any given area.
2) The selected area must include both the segment start and the segment finish
3) Long segments would show up only when you zoom the map out. But zooming the map out also includes a lot of other segments, that are very likely to be more popular. In general shorter segments are more popular because more people are likely to match them.
Basically that keeps longer segments nearly invisible.
Furthermore the existing filter for running segments only allows to select flat segments vs. hilly vs. steep, but that doesn't actually work correctly. For example if a segment goes to the top of a mountain and back, that segment would be considered flat (0%) because the start and the finish are at the same elevation. Similarly, flat segments include all downhill segments even if they are extremely steep, for example -30%).
The mobile app has a different segment search tool, but it is also finicky and even frustrating to use. I find that it refuses to show me some segments no matter how popular they are. Even if I zoom in all the way and I know that there is only one segment in the shown map area, the mobile segment explorer may still often refuse to show that segment.
What I really want for both the web version and the app version of the segment explorer is to enter a few parameters (e.g. the distance and the elevation gain) and see all segments. Even if the number of segments is too great, there should be a mechanism to discover them all, especially the longer ones that aren't easily discoverable now.