Xcode tracks files with an XML "project file." So if you add files in one branch and remove files in another branch, it often puts this project file into a conflicted state. Resolving conflicts in this file is really difficult — lots of confusing paths and SHAs all mixed together. If you don't resolve conflicts correctly, the entire project refuses to open.

I work on a project with 3 other devs; an xcode conflict is an "oh, bummer" moment, but not a hairpuller. 90% of the time we've both added files, and the merge is as easy as "choose both."

It's still the sort of thing where you want to check out, change the project, and check back in quick to avoid conflicts. I can imagine it getting worse with far-distance branches though.

It's not too different from a makefile, except you'll get a better error message when you muck up your makefile.