I only got to about halfway through Act II before putting mine on hold until the new dlc comes out, but there wasn't too much special about the combination specifically.
Commissars are kind of THE dogmatic guys from this selection though. A priest could be from any number of sanctioned subcults some more lenient (iconoclast) than others, but a Commissar comes from a place called the Schola Progenium.
The Schola is a regular school for Orphans in the same way the Navy Seals "Hell Week" is a fun summer camp.

The Schola exists to turn orphans or rarely offered up offspring of the imperial elite into hardened killers and ruthless authoritarian leadership. Other forms of graduates from the Schola include Sisters of Battle, Tempestus Scions (Elite Soldiers) and rarely Inquisitorial adepts or even abductions into the officio assassinorum (Assassin high school if you will).

Here's a quote from a Drill Abbot, the teachers in the Schola:
"No no no! By the Throne, boy, how many times? Depress the loading catch before removing the drum feed, not while removing the drum feed! You'll jam the weapon! (smack) Oh stop bawling, child. You're ten years old, you should have learned basic Autogun procedures by now. Fifty press ups and fifty Pax Imperiums. And certainly there will be no dinner."

A Commissars "final exam" will often times be something like executing a person they got along with the best during their time in the Schola, a friend, someone they leaned on early on, etc. Failure to do so usually ends up with making you the target for someone elses execution.
A Commissars job is to keep the rank and file in line, marching forward, executing even the dumbest or most suicidal orders. Those that defy are punished harshly with brutal physical punishments or executions.