The main disbelieving I have about the series is that the FBI never got a profiler to write up a profile of any of the serial killers they were trying to catch (the Icetruck killer of Dexter himself)
All the profiling is done by the detectives themselves, who are obviously not trained for the job.
For example, I think a trained profiler should have been able to figure out the Bay Side Butcher worked in law enforcement just from the bodies and M.O. (He managed to find killers the police couldn’t track down, obviously he should have had atleast the same resources)