I fired it up on a Git export of a Fossil repo here, and it's missing features of Fossil's web UI timeline view:
1. Cherrypick markers
2. Diff arbitrary versions (it shows only diff from previous)
3. Branch coloring
4. Hyperlinks to produce new timeline views: click author to get list of commits by that author, click timestamp to get a new timeline surrounding that point in time, etc....
5. Integration with other web UI features. For instance, you can create a Fossil wiki article attached to a commit from Fossil UI, but you can't attach a GitHub wiki article to a Git commit from gitk.
For another, Fossil has a feature to produce zip and tarball downloads of particular versions, which have links in the commit info page, which means visitors to your project don't need to clone the whole thing and roll it back to that version manually to get a single version. That's integrated into the Fossil web UI timeline: click a version, click Zip, done.
6. A modern browser fits its platform and offers more UI affordances than the mid-1990s Motif-inspired Tk. (Better copy/paste behavior, font rendering, zooming, native controls, etc.) Web UIs get a lot of angst these days, but like them or not, they're native citizens of the host platform these days in a way that Tk is not.