It's always been a competition (seemingly) amongst designers of opioids to see how few carbon atoms they could use in their structure. The common target was usually M potency. I remember that they made quite a big thing about tapentadol over this.
I hope you see what I'm getting at. Carbon is the scaffold of organic drugs so the fewer Cs, the lower the complexity. It doesn't follow that it means that it's the simpler to make, but it does mean simpler final complexity.
I'm not getting into the subjective (side)effects such as with tapentandol (in particular) but I've not seen a simpler 'morphine range' opioid with 14 carbons before and I doubt it's possible... but I would like to be proved wrong...