Unfortunately, Mercurial is somewhat less flexible than git when it comes to using branches to separate ongoing development work (which is a workflow encouraged by using Jira/BitBucket) - there is a tight relationship between branch names across repositories and it's impossible to delete branches once they have been created or merged. Sure, we could have probably used Mercurial bookmarks for this, but they are not well supported by the Jira and BitBucket workflows we are using (and are more designed to be used locally, not across multiple repositories).
In addition to that, we have a growing developer/contributor base that simply is more familiar with git than Mercurial nowadays. Switching to git could potentially help us attracting a bigger number of developers.
These were the main reasons for our decision to convert. Thankfully, with the help of the fast-export utility, the actual conversion was straightforward and painless. It automatically renamed the former default branch to master, to be conforming to established git practices. We will revisit the branch naming conventions in a following step.
There was some cleanup work involved afterwards, to remove all the obsolete branches that were created by Mercurial when merging pull requests.
Additionally, all pending pull requests from the previous Mercurial repo had to be re-created, but that provided us with a good opportunity to clean up the commit history before submitting them. Going forward, we intend to also make use of the more advanced features of git like rebasing or squashing change sets, to have a cleaner revision history with less noise.
We're now in the process of updating our documentation and build scripts to support this change, but this should not take us too long to resolve.