Think about the jobs lost if CSA took Philadelphia and razed its structures to the ground. It doesn't just add up to national morale and local modifiers on loyalty and money/supply/wsu production. You'd have riots, famine, disease, high crime, tax revenue lost... come to think of it, it's what Sherman did himself too and this would require heavy garrisoning as well as political and economic choices to counter the effects.
This is another option CSA is missing in this system.
As of the AI since this was originally an AI thread, my very first mission with Schurtz' cavs is to sabotage the rail lines in VA. I manage to sabotage all of them and the AI sometimes doesn't bother fixing them. Conversely, the AI doesn't send much of infiltration forces and raiders up in PA and WV. I think I saw at best ONE rail sabotaged while me in comparison have sabotaged dozens of rail lines. This shows the AI doesn't care about sabotaging (and we know the engine misses the effects of this sabotage, with the except of the costs to be paid to repair the lines and the fact you've got to travel on foot which already should be the NORM) and cares very little about repairing lines.
Another thing I noticed about CSA AI is that by event I had 2 CAVs in Dallas. It's DEC 1861 and CSA hasn't even tried to take it back. Theoretically, I am surrounded there and I have no way to support them. It's an easy task and the AI has increased FOV/Activation, it just doesn't seem to care.
... as I said... I've seen enough to put me off the game already and I am truly sad because this could very well be a masterpiece if those deep mechanical issues were addressed.
As of the AI I wouldn't bother much because I know this is so complex it can never really be smart enough to outwit the player. On the contrary with the exception of these 2 aforementioned problems I think it's a surprisingly good opponent. It loses this bad only because of the union's "magic" powers.