Aren't mechanical and moving parts less reliable than 
    electronics, in general? 

I don't dare to answer this in general, maybe someone more knowledgeable can offer his or her opinion. In our case the alternatives were:

  1. Simple gear and constant velocity three-phase motor[1]
  2. Two axis mounting and two stepper motors.

First option has less parts and should be more reliable in theory. That being said: I only know of commercially available solutions of the second kind. Maybe this has changed in the last ten years, but our solution never made it to market.

[1] Industrial three-phase motors are real workhorses, produced in large quantities since ages, relatively cheap and incredibly reliable.

You can think of a mechanical solution as a straight line program with no conditionals. The servo solution has a feedback loop that will ultimately face an upset in the feedback loop, a broken sensor, worn brushes on the servo motor, over heated power transistors, etc. Both solutions will suffer from fatigue in the long term, but an active solution with electronics and a servo will suffer from a element failing and possibly causing the active solution to destroy itself.