Bumping this thread since there’s already been some good conversation on this topic.
I have found it pretty difficult to use the search within the Mantis Tracker to find relevant results. It’s hard to quickly find if there is already an issue for a bug that I observe.
ITK recently moved from their issue tracker (JIRA) to GitHub issues. See this post on their forum. Here is another post they had about using GitHub labels. For the most part I think Github Issues, Labels, and Milestones could replace a lot of primary functionality in the issue tracker. There isn’t default assigning of issues, but maybe some github apps could supplement any missing features.