My experience with hg-git has been that it is not always a great citizen with git repos and it can cause something of a mess. (This is a few years old now, because like the rest of the world I switched to git; it may have improved in the meantime.)
It's because mercurial is legacy, like darcs, bzr, monotone, and the other zoo of "git-alikes" that were big in 2008. Now it's just like MacOS Classic in 2000: there are a few die-hard zealots who claim that the interface is better in some indescribable way, and everyone else has moved on.
Mercurial is very important for Facebook, Mozilla, Google, and others. The disparaging "legacy" monicker might be a good way to feel justified to not have to learn about it, but it's not dying software that is no longer getting updates.
Speaking as someone that works with Mercurial on daily basis, and also makes/sells software that supports Mercurial. I can tell you it's far from beeing legacy.
What determines if a technology is considered legacy is not if there exists someone out there using it, but rather whether there are people actively adopting it at present. In this regard, Mercurial has slowed down and Git has clearly won the game.
Not trying to start the classic hg vs git debate, just saying that it's great to have the option.