Parthia never even tried to attack me, and remained for the most part a 2-3 regions power, even when they had the chance of striking me. Saka doesn't expand much too, but conquering them is another thing entirely... they have those heavy horse archers that perform good in almost any situation and they often killed my generals.
Based on my experience, the factions that actually expand succesfully are: aedui, carthage, ptolemaioi. Factions that should expand a lot but they don't: romani, macedon, seleucids. Factions that are not expected to expand much and they don't: almost all the rest.