The AI is okay at offence. It will build up a strong force and then strike. In that sense, offence is pretty easy to design, as a programmer - you don't have to find a way to think very much about what the other side is doing. You basically only need to think about the path you'll take, to avoid AT or anti-inf. I'm pretty sure the AI knows where all things are at all times, even stealthed units (It just won't aim at stealthed units), so the information needed is available.
Defence, though, the AI doesn't really do anything, not that I've seen. Defence is mainly about forming a mobile reserve and using it to strike (or better yet, flank) enemy offensive concentrations, plus a thin outer line to let you know where things are coming from.
To what extent can the AI be controlled/modified, in BKM?