The missile AI seems to target Groups now, as that's more efficient, but it'll go after single targets it they are high-priority or simply the only things within range.
On a missile-related side-note, I had a pleasant surprise the other day. I was playing as Man against Ulm, and was rather surprised when the AI cast Arrow Fend since the opposing side had no missile Users. Right after that, my horde of Longbowmen opened fire on the clashing melee line, inflicting no casualties because of the Arrow Fend but absolutely demolishing the opposition. I'd never seen (or maybe just never noticed) it doing that before, and it was beautiful!