By default, ROOTConfig.cmake appends the installed "cmake/modules/" directory to CMAKE_MODULE_PATH. With certain modules in that directory overriding core functionality and/or supplying FindX cmake modules, this can make build behaviour highly dependent on the ordering of find_package calls in a client project (and difficult to debug...).
It might be better to
- Only append to CMAKE_MODULE_PATH in ROOTUseFile.cmake (or not at all and simply include RootNewMacros.cmake by full path)
- If FindX.cmake modules are to be installed (I'm aware of
ROOT-7572, but this appears to not be present in at least 6.04.06), then they should be partitioned into another subdirectory which is only added to CMAKE_MODULE_PATH by an explicit function/macro call.