It adds a scrollbar or else some kind of cue that scrolling is available. Horizontal scrolling in desktop feels unnatural personally. And finally, the oval links look like tags in an article, not navigational controls. That last point could be adjusted by squaring off the buttons, by adding a symbol between links indicating a nav path, or something like that.
Not to mention when using a laptop it would be easy to unintentionally trigger a “navigate back or forward” browser event when scrolling horizontally. Such an annoying browser feature.
