First, it's slow and bloated. According to Firefox's dev tools, the main project's page takes 15 seconds to load, transferring 3.6MB of data in 62 HTTP requests. This is with a 2015 MacBook on a 150Mbit connection.
Second, it's a UI nightmare. The most common action for a story (expand details) is a 6x9px light-gray triangle with a 17x24px hitbox—not exactly the easiest target to click on. And Pivotal Labs must offer free LASIK to employees, because the default text size is absurdly small. On a non-retina screen, one is forced to use the "Projector" view. The UI offers few affordances, and many are misleading. eg: Each story's "Close" button doesn't modify the story in any way. It simply navigates back to the project overview page. The whole product is fraught with these sorts of issues.
It's telling that Pivotal Tracker is usually imposed upon dev teams. Few devs freely decide to use it; they're fine with a simple issue tracker. It's management and product teams that want the categorization, time estimates, and hierarchical organization of the borderline-chaos that is software development.