The offset tracking module is basically searching for matching patterns in both images within a small given area. If glaciers are covered by stones or other gravel, these patterns are preserved, even if the glacier is moving.
For landslides, surface patterns often completely change (trees break, surfaces are overturned or tilted) so the matching algorithm has no chance to find suitable pixel pairs.
I would say it works if the landslide moved a large part (several hundred meters, as S1 has only 10m pixels) which didn't change at its surface.
If it is a landslide like shown below, you don't have any chance.Image source