Upon setting up Xcode server on Yosemite 10.10 I ran into a couple of curious issues that were causing frustration and didn't seem well documented. This won't be well documented but at least here are a couple of hints should things go wrong...
First up was git. As is usually the case I stated with a local repository and only later decided to use Xcode Server. Once I enabled Xcode Server it seemed to work. But upon adding the remote git repo I ran into strange errors. It repeatedly said the repo already existed on the server but also said there was no functioning remote repo. I'd chosen to connect to the repo via ssh:// and it turns out there was a conflicting entry in my known hosts file. With that entry removed all proceeded as planned. Moral of this story is if you have issues connecting to git from Xcode be sure to try it from the command line and see if there are any errors that aren't being surfaced.
The next issue was after the first run all the simulator builds failed. The error was something like this: Read more about Xcode server and continuous integration for iOS