When trying to register a new host via global registration with the default template to a Katello instance, registration fails if a REX interface is included in the request.
I could not directly verify this, but from my analysis this only affects Katello instances and not plain Foreman due to differences between the register_host() and register_katello_host() functions in the default registration template.

The error message is:
Interface with the '<interfacename>' identifier was specified as a remote execution interface, however the interface was not found on the host. If the interface exists, it needs to be created in Foreman during the registration.

• Have a host currently not registered within Katello
• Generate a registration command with "Remote Execution Interface" set to the name of an interface on the host
• Try to register the host with that command