It becomes just as much of a PITA importing projects github and maven for example as well.
We actually ended up sticking with svn and trac and doing manual mirrors of every source dependency we pull in to protect our asses. SVN is kept only due to externals support (even though all our externals are internal!). Our head rev is over a quarter of million revs, 37gb of data and the repository has been online since 2006 to give you an idea.
I'm aware of github enterprise btw - its actually easier to build our own using trac as we can support it end to end, its a crap load more configurable and we can scale it up and run failover nodes easily using svnsync and pgsql replication. We have our own plugins and reports plugged in using reportlab as well. And its not atlassian's crap either.