I can only speak from my own experience but I designed my 'Battle of Italy' scenario around the 189 unit grid and found the AI sufficiently aggressive to make the game challenging for the human player.
In my case, I was using two particular techniques that undoudtedly helped:
- through the use of the lua language I permanently set the Attitude of the warring parties to 100 dislike (and as I recall the same can be done with macro) and the war status at on, and
- with a judicious use of GoTo commands, pushed the AI combatants towards the enemy whenever and wherever appropriate
Similarly, I found that in Techumseh's '1937' scenario representing the Sino-Japanese 1937-1945 conflict, and which also uses the 189 grid, that the Chinese controlled AI could be quite belligerent.
Both these scenarios had the advantage of only having 2 opposing factions at play so this assuredly aided in keeping the parties in a state of perpetual conflict (though in my Napoleon scenario, which used the 127 unit grid, I was also able to control the state of war between the 7 different powers by modifying the war status and dislike levels whenever France ended up at war with a member of a Coalition and they could be quite warlike under these circumstances).
Of course, in the base game itself some of the tribes have a more bellicose stance than others (like the Russians and Mongols), so it helps to select these as your primary aggressive nations in any scenario you want to make.
I cannot say if the same can be said for your typical Civilization II Empire building scenario, and whether the 189 unit grid is an issue there, but it's been my observation that a scenario designer does have some tools at their disposal to control the combative level of the opposing factions.
EDIT: though 1937’s rules file is set up to support 189 units, the scenario only actually uses 127, which as I mentioned still has an aggressive AI.
John Petroski’s ‘Hinge of Fate’ does use 189 units, and though the war making burden of this scenario lies almost primarily on the human Axis player (it’s up to Germany to conquer Europe), I recall while playing it that the AI powers were not exactly push overs.