If actual_departure_time==actual_arrival_time that would mean we received information that the flight departed, but we did not receive any information on an arrival and gave up waiting for an arrival message. This is typically indicated by result unknown or something similar on the FlightAware site.
adhoc==true while en route means that we have not received a destination and an eta.
adhoc==true after the flight has landed would mean that we did not receive receive or interpret an arrival airport.
adhoc==true while en route then adhoc==false after arrival (for the same flightid) means we interpreted the origin and destination. We would now have origin, destination, and eta to satisfy the conditions for the flight to not be adhoc.
Your statement regarding adhoc is correct. As far as frequency goes, it depends on what your tracking. Often times cargo flights that fly as the same callsign as the passenger flights, will come up as adhoc, but we may infer the departing and/or arrival airport.