While the description for bf:Identifier (http://id.loc.gov/ontologies/bibframe.html#c_Identifier) is "Token or name that is associated with a resource, such as a URI or an ISBN", its subclasses include bf:Isbn but not bf:Uri[1].

For now I've interpreted this as "just use bf:Identifier if you have a URI," but if that interpretation is correct, it might make sense to clarify the description.

Example:

<http://example.org/2335409#Work> a bf:Text, bf:Work ;