Tried this and Real Strategy (which emphasizes unit building) yesterday on immortal with the now very strong Mayans, shared a continent with the Scythians.
When they declared war, they brought an absolutely scary stack of units, that would have been devastating in the open field. The Ai is really not bad at using this tactically, quickly pouncing on isolated units.
However, it did highlight the problem of overpowered city defenses (and you can stack your cities + encampments too, of course), because I could still defend my cities relatively comfortably, and the Ai still tends to naively send isolated units into enemy lands itself. I had to be prepared though, my walled encampments saved my border cities multiple times. And after I mowed down the first Scythian wave (the Mayans are obviously good defenders), they didn't just surrender, but came up with a stronger second one.
So it does make things more interesting, maybe more dangerous, not convinced yet that it's harder. It definitely enables the AI to take other AI's cities, so I might still get problems with a snowballer later, let's see.
