Lets be honest. All seasons of Dexter have had a tonne of "coincidences", unrealistic turns of events, etc... But the episodes moved at such a fast pace, there was never time to dwell on them. The first four seasons of Dexter moved at almost movie pace in terms of how much happened in each episode. 12 episodes of dexter contained more plot and development than multiple seasons of regular TV shows like 'Lost'. But this last season, the plot stopped moving quickly. Each episode only had one significant event. It became just like any other TV show's pace, and so each plot discrepensie or unrealistic event had plenty of time to stick in your mind and dwell on.
This last episode moved past that and had a much faster pace - a good note to end the series on.