Another difference that may matter for some: on block based CoW FS, shared executables and libs will not be shared among containers, because file cache is file

Another difference that may matter for some: on block based CoW FS, shared executables and libs will not be shared among containers, because file cache is file based not block based. Using Overlayfs - or other file level CoW storage, when one starts several containers from the same image, shared libraries and executables will be loaded only once(so will be actually shared) across containers.