It seems like Xcode 9 beta does not like "~ipad" in filenames for .xib files. When selecting the new build system (for a project, that is compiling successfully with the old build system), it errors out with:
unable to build node: '/Users/dev/Library/Developer/Xcode/DerivedData/proj-aiatvieseyfwjkfqncikqcohpdsn/Build/Products/Debug-iphoneos/proj.app/Controller~ipad.nib' (node is produced by multiple commands; e.g., 'adf8a93da84709e14beaf35ec6d6d21c02c34dc4669b6bd1e18a027ff75b53b5:CompileXIB /Users/dev/git/proj/Resources/Controllers/Controller.xib' and 'adf8a93da84709e14beaf35ec6d6d21c02c34dc4669b6bd1e18a027ff75b53b5:CompileXIB /Users/dev/git/proj/Resources/Controllers/Controller~ipad.xib')
Should I rename them or is there another solution?
It's a known bug in Xcode 9 beta. There is no workaround for the moment.
"XIB and Storyboard files which use platform-specific naming conventions (e.g. “MyView~ipad.xib”) may cause the new build system to emit an “unable to build node” error. (32409101)"
So YES, you can try to remove the "~ipad" and just execute it on iPad simulator.
Open radar: New Build System fails to build project containing universal xib