There is a time when Executive Privilege can NOT be used....during an Impeachment Trial.
Depends on what is being subpoena’d. If it involves national security matters, or if it involves attorney-client matters, or such, it would require a SHOW CAUSE for the need. Fishing expeditions can never be any good and that is why we have SCOTUS.
Otherwise any partisan House of Reps can begin a fishing expedition against any president they hate, and just impeach him and negate the votes of half of the country.