Docker & hosts filesystem

trying to understand a basic concept regarding docker…
I have looked for volumes, data storage, etc topics lately and got the impression the filesystem your host is installed with. It goes so far that I read somewhere it can be close to fatal whether that was btrfs. As far as I understand this it due to some incompatibility between the container internal aufs and btrfs

Wouldn’t that practically mean that a container is not as isolated after all? Furthermore … which filesystems are save to use? ext4