Well allow me to elaborate - have you ever merged two nib files? With today's variant you merge XML. With yesterday's variant you simply don't do it because it's not possible. So you're merging an obtuse XML file at best. In my mind that's painful versus merging clean .h and .m files where we know what's going on. So he's using colorful language to describe such situations - A "nightmare".

these things work well-enough in very small development teams.

We're a two person development team who sits in the same 5 foot radius and we've had to merge xibs semi-frequently. It's not overly viable to ask the person next to you what files they plan to work on in the hour, because that can be difficult to determine in advance. Additionally once I upgraded Xcode before my partner, and suddenly our xibs were out of sync. In terms of one person working on the UI, even if that's the case, the other person might change an outlet, or create a non-UI object in the xib. It's complicated to coordinate changes because the breadth of features (as they appear in the codebase) is unpredictable.