The existing design on Hordes is that they should reform or die, pretty much. You can argue that design should change, but leaving an exploity workaround (recruiting in foreign cores) is not the way to go about it.
I'm all for more challenge, but stripping away a gameplay feature and leaving nothing in its wake is a little rough.
But why would the hordes get stuck with one level of unit when the native american have dozens ?
It's all well and good pointing out what makes hordes weak, but what I think we need to do is make practiceable suggestions about how to make them better.
Err... it wasn't an exploit. The mechanic had to have been intentionally and explicitly included, so it was working as designed.
And it made sense. Recruiting local military should look like the military of the *people* who live there. If anything, you should *only* be able to hire those units, and not your own national units - the local population doesn't completely switch over to nomadism in 23 months. If I was going to make a change, I'd use culture and foreign cores as the sole determinant of units available to hire, and national units only become available when it's *only* your own core on the province. (Enough time has passed that the populace has become fully integrated into your way of doing things).