When a political party secedes from Rome (unsure how this plays out in other factions) they are able to recruit any unit they want, regardless of the state of the faction they just seceded from.
I would think that Senate Secessionists would not be able to produce an end game unit when they secede after 30 turns. An example: Secessionists recruited Praetorians while I'm still running around with Princepes. This "bug" destroys balance and immersion.
Hope to see a fix or someone defending this "feature" if it so happens to be one.
All the best.