Hordes using that was always more of a bug than a feature, since only players did it and it was very obviously a loophole around the intended design of units not improving.
Not buying it. Every time there's a feature, it's a feature right up until it is suddenly defined as an exploit. That dance was done with military access too. Recruiting from foreign cores only had some significant downsides:
1. Limited to one unit type once cannons exist (IE the entire time you'd actually want to do this), often the one horde gets 0 unit bonuses to improve combat ability (junking their 20% bonus which is tied for 2nd largest during long stretches).
2. No mercs at all after a while, except maybe to cover colonies or siege. Considering the no cost-to-reinforce and low maintenance from NIs, losing the ability to run merc-heavy armies and ignore manpower as a constraint at a large discount is a big opportunity cost.
3. No unit upgrades. Any time it was time for an upgrade, you had to keep obsolete units or delete them, and use your manpower pool to retrain entire new armies, often in territories with damaged or limited infrastructure due to conquest.
4. 75% tech penalty. There are only two major regions in the world that had a worse malus to tech. Good rulers don't offset what amounts to >1 point/category weaker ruler even compared to Muslim tech. This had serious consequences on idea progression.
So, this "exploit" (which was almost never/possibly never called as such by a developer for over a year since its implementation and regular use) actually comes with considerable downsides, including large windows of vulnerability while rebuilding armies from a limited number of provinces, no ability to reinforce mid-late game armies with mercs, and weaker idea development. Yes, you got some unique mechanics in return...and for 7 patches, it wasn't an exploit, now suddenly it is?