In my experience fast-import/fast-export can fail due to out of memory.
This tends to happen when large binary files are present.
This has long been raised as an issue, but last time I remember checking (~september 2010) it didn't look like it was going to be addressed. At one time, I wasn't able to migrate my bazaar repository with history to git because of this.
I believe I may later managed to do so by switching to a 64 bit distribution.