I have Xcode 9 installed, and have also updated High Sierra. I've got no problem compiling for IOS, but am getting a flash compile error 'Error: The input swf newgrounds.swf' when trying to compile for browser or flash player.
I've not tried to test on the iphone x simulator - will have to take a look.
EDIT: just tried to compile for simulators from inside xcode, and none of the simulators work for me. Compiling for a connected device from inside xcode does work, archiving for up-load from inside xcode works. Simulators aren't listed in stencyl dropdown.