The limits to political power have been previously noted when megacorp first came out and nothing has changed yet, so I don't think it's fair to say it's developer oversight.
To be clear on how the game calculates the planets political power and stability with a stratified society (with only 2 rulers & 10 slaves on the planet for simplicity sake):
The rulers when happy will only stabilize the planet with 2 political power into the calculation, but when they are unhappy they will use all 10 of their political power to make it unstable, this basically means that rulers are limited in making the planet stable but can quickly cause the planet to become unstable when displeased.
The slaves when happy will stabilize the planet with 0.25 political power into the calculation, but when they are unhappy they cause 0.5 political power to make it unstable.
So for example, 75% happy rulers and 25% happy slaves will get you:
2 rulers x 75% happy x 2 PP and 10 slaves x 25% happy x 0.5 PP for 425% in the numerator.
2 rulers x 2 PP and 10 slaves x 0.5 PP for a 9 in the denominator.
So you have stability of 47%
If you flip them so rulers are 25% happy and slaves 75% happy you get:
2 rulers x 25% happy x 10 PP and 10 slaves x 75% happy x 0.25 PP for 687.5% in the numerator.
2 rulers x 10 PP and 10 slaves x 0.25 PP for a 22.5 in the denominator.
So you have stability of 31%
Slave processing centers are interesting because their PP decrease to slaves only works when slaves are happy, but since slaves tend to be the least happy pops it still works in a generally positive manner.
So for example, without a slave processing center, you have 100% happy rulers and 75% happy slaves will get you:
2 rulers x 100% happy x 2 PP and 10 slaves x 75% happy x 0.25 PP for 587.5% in the numerator.
2 rulers x 2 PP and 10 slaves x 0.25 PP for a 6.5 in the denominator.
So you have stability of 90%
So for example, with a slave processing center, you have 100% happy rulers and 75% happy slaves will get you:
2 rulers x 100% happy x 2 PP and 10 slaves x 75% happy x 0.0 PP for 400% in the numerator.
2 rulers x 2 PP and 10 slaves x 0.0 PP for a 4 in the denominator.
So you have stability of 100%
I think Nerve Stapling has been fixed so that those types of pops have 0 political power, so a slave processing center would not affect the PP on the planet or the stability calculation, and those kinds of slaves neither increase nor decrease stability.