I've used it in a product for a couple of thousand repos. The big problem is architectural. Each branch is serialized into a single bundle file. There is no structural sharing of git objects. So each and every branch will download the full history from scratch. So changing branches is as expensive as a fresh clone. If you combine this with a real user's desire for images/diagrams of any kind, then boom, massive slowness.
There are also two concurrency bugs which the maintainer refuses to acknowledge.