I've used both, and I can only imagine your complaints about JIRA's UI are due to how it's been configured for your use.
The out of the box user experience is the best I've seen for a web application. A robust yet easy to use 'search' feature is at the heart of many things, like the scrum and kanban boards, as well as widgets that can be put on your own personalize dashboard. You get a nice visual designer for setting up workflows, and you can easily create new fields for issue types and have workflow depend on those fields.
I also find Stash to be well thought out and easy to use if your use-case is a pull-request workflow tied to JIRA. Sometimes it's not intuitive how to navigate around if you're just trying to view source, but that's not the selling point. If you're trying to navigate source history, use your local source control client, that's what it's built for.