Redirection option will be good when validation fails
Use case
when entity reference is used to create a new node with repopulated field referenced node id. there is no way to prevent creation of new node without reference.i.e simply user can go to node/add and create . but form will open.
here we can use field validation to prevent a empty form being opened
here is my suggestion
set the validation on reference field as "required" use field permissions to custom and select only "admin"
this works fine to prevent creation(saving) of new node, but cannot prevent for opening . if we have an option to redirect if validation fails we can give a redirect path and message can be displayed on the redirected page along with a message "you cannot create this node"