Playing as West Rome, it becomes a little frustrating that every new unit, replaces an older unit, even if the two units can and should coexist.
For example, why must Limitani be entirely replaced by Comitatenses? Even though those two spear units can and should fill entirely seperate roles? Comitatenses are 'professional' spears, and Limitani are a decent 'militia' unit? Further up, Comitatenses then are replaced by Auxilia Palatina...why? Even though Auxilia are rather expensive.
Essentially, what is an otherwise large Unit tree becomes very, very restrictive.
For example, why can't I have a mix of decent spears, mid-level spears,
and cheaper spears? Limitani as cheap front line units, with Comitatenses and Auxilia held in reserve, but with each unit -replacing- its predecessor this becomes impossible.
Not only historically, but also gameplay wise these units can fill very different roles due to their differing cost and states. Why the need to replace one with the other?