A forthcoming update to Total War: Warhammer will enable AI factions (with some restrictions) to recruit Regiments of Renown to their cause. At present, they’re unable to do this.
The Regiments of Renown were added by the recent Grim & the Grave DLC and (presently) consist of various named Empire and Vampire Count units with boosted stats and special properties (such as Vanguard deployment on a troop type that usually doesn’t have this trait). According to Creative Assembly, in the next update “The major AI factions in a race that has Regiment of Renown units will now recruit them provided the player is not a faction within their race.”
In other words, if you’re playing as Vampire Counts, you won’t see any other VC factions using the Chillgheists. But when fighting the Empire, you may well see one of the Elector Counts bringing some Regiments of Renown to the battle. This system will presumably apply to future Regiment of Renown releases for races like Greenskins, Dwarfs, etc too.
It has not yet stated when this next update to Total War: Warhammer will be released.