One big difference is that if you don't declare a resource it doesn't exist anymore. With chef, puppet and friends there's a big gray area of things that are on

One big difference is that if you don't declare a resource it doesn't exist anymore. With chef, puppet and friends there's a big gray area of things that are one the machine but not managed by the tool. In practice there are always things lingering on old machines that are not described by the SCM.