I think AI, in this case, is overkill. Create ~10 common enemy class and spawn them based on your strategy history, equipment and game pace. Each and every enemy would act differently like if you were more on sniping side, you would get snipers and fast units spawned. With mediocre AI this would make the game appear as having very good AI to regular player.
A large part of the predictability is how the units come in at pre-defined locations and/or based on pre-defined triggers. There's very little randomness in this department.
If that was more unpredictable, if each death switched things up slightly, you'd have a far harder time gaming the system.
Like you observe, that might also make the AI appear smarter since the flaws are less obvious.
