I suspect that, coding-wise, there is little to no difference between assassination plots between rulers and non-rulers, in that it moves forward at the same place. That slow pace is good for rulers (since you potentially get options to shut the plot down), but inconvenient for quick adjustments. It's worth remembering that assassination is not the same as proscription (the practice coined in Rome for dictators like Sulla marking individuals for death). You're having to quietly do away with an opponent for the very reason that moving against them publicly might end up with a Civil War.
With that said, I can absolutely see a set of innovations either specifically to speed ruler assassination schemes, or to improve speed/efficacy of ruler schemes in general (making "assassinate" or "prove legitimacy" go faster, while "siphon money" gives less corruption or some such).
