The fact that Gitlab Issues aren't a replacement for Jira is a feature IMO. Jira is surprisingly awful at its main job. For example you can't have more than 2 levels of parent/child tasks (compared to Phabricator which has no limit). Changing an issue to a task or vice versa goes via a complex batch update mechanism. Over-configurability means you end up with a gazillion different task states (Done, Resolved, Finished, Closed, ...). You can't do basic things like reorder the backlog based on priority.

Perhaps most critically the interface is just insanely slow! It regularly causes waits in sprint meetings while we wait for someone to drag & drop an issue or for the page to load.

The only reason it's popular is because PMs like to make engineers do their job for them so they can just click a button and get pretty graphs that they can copy & paste to presentations.