No, they aren't any more difficult to defend than any other offshore platform. They do interfere with long-range land-based radar in a way that is problematic with the emergence of shipborne drones.

Are they shutting down offshore oil drilling too?

Order of magnitude increase in difficulty to defend a wind farm vs an oil rig. Wind farms are dispersed, not continuously manned, harder to monitor/enforce a 500m maritime safety zone of exclusion, have a greater attack surface (subsea cables, substations), and are easier to sabotage with plausible deniability