It's much inferior to the sequel title, Ultimate General: Civil War, which is hands down the best real-time linear battle tactics game I have played. They learned a lot of lessons from UG: Gettysburg to the point that UG:Gettysburg feels like it was just the proof of concept or early access version of UG: Civil War.
They made the units much easier to control on the field of battle compared to UG: Gettysburg. For example, you can shuffle brigades over to the left and right without having them wheel and expose flank whenever you want to dress the gap in your line or extend your line, and they made it so artillery battery can shoot over the heads of infantry and cavalry. They made it so a brigade doesn't have to be fully halted to start delivering a volley, and generally made the unit behavior much less rigid and more flexible and natural to simulate how the lines of battle were a little sloppier, more practical, and not like a strict parade ground formation in combat
They also added a whole lot of good mechanics, on and off the field of battle, that really help to get immersion and realism stronger. They added different condition states for morale and physical exertion of units (ie a unit that has been "blown" from running at the double quick to get to the frontline will behave differently from a "whipped" but otherwise fresh unit) that are easily readable, they added ammo supply for all units that can run low in battle if not resupplied. Instead of just having a binary state of "whipped to the point of uncontrollably fleeing the field" and "responsive", they have varying states of cohesion and morale for units. A unit can be responding to your movement and firing orders but struggling to coordinate volley fire, a unit can be responding to orders but inefficiently, a unit can be phasing in and out of responsiveness while still not breaking, etc. You can detach skirmisher companies from the main body of troops, to more accurately simulate the standard practice of throwing out a picket line to recon forward of the main body in an advance or to serve as a tripwire for the main line of resistance in defence.
They added a bunch of new soft and hard cover/concealment types to the battlefield terrains, and even added entrenchments and field/siege fortifications that take greater prominence in the late war campaigns. The addition of entrenchments and fortifications really changes the way the game plays in the late war campaigns because it shifts away from the clean linear, set pieces battles of the early war to the bloody grind of trench warfare and high-casualty close infantry assaults like how it was at Spotsylvania, Cold Harbor, Petersburg, etc.
But they also modified the entire scope of the game. Instead of just being a "division" level simulation, where you are pushing brigades around the map, they made it so that it is kind of like an "army" level simulation, where you are leading the same multi-corps army through all the major Western and Eastern Theatre campaigns of the War. You get to name, create, reorganize, or disband the various infantry/cavalry/skirmisher brigades and artillery batteries in your army, choose which small arms and guns they get from your arsenal supply, and choose which officers will command them, then transfer the brigades and batteries between divisions and corps to optimize their synergy on the field. The fact that the different brigades and batteries develop veterancy status from their combat performance in past battles does a good deal towards making it feel like a much more individualized and personalized army, as you will inevitably end up developing some high-morale elite "assault" brigades that are hardened veterans of many battles and some less reliable second line units that will break more quickly due to suffering heavy losses in previous engagements. As you can change the names of your different units and review their past combat performance on a battle-by-battle basis (who was the commanding officer, what weapon were the men armed with, how many men did they field, how many did they lose, how many did they kill, etc), it is very easy to get attached to your elite units and feel a kind of loss when they get savaged in a bad engagement or sacrificed to shore up a collapsing flank.
A lot of those role-playing elements really make it feel like a much better overall campaign simulator. The types, quantity, and availability of different types of weapons and replacement troops will change over the course of the war for both Confederate and Union armies, and they are also directly affected by your performance in the preceding battles and campaigns. You can expand your artillery and infantry arsenal by capturing or killing enemy artillery and infantry/cavalry/skirmishers, which "drop" a percentage of battlefield salvageable arms, modified by whether the enemy units were captured whole or killed piecemeal and whether you hold the field at the end of the battle. While fighting the big linear setpiece battle, you (or the enemy AI for that matter) can even send cavalry or skirmishers behind enemy lines to capture their ammo supply train, if it's not sufficiently guarded, and bring it back to your own lines for your troops to use or just to save it towards the next campaign. The opposing nations' armies have a set manpower pool that they can draw from for constituting their brigades and divisions and corps, and you can tangibly bleed down the AI opponent's "national" manpower pool by inflicting high casualties among their veteran units during battles so that they have to fill their ranks with green conscripts, which will limit their units' effectiveness in future battles.
So for example, in a successful Union campaign, by 1865 you should be facing severely undersized Confederate brigades comprised of scarecrow veterans because you've killed off the cream of the Southern armies and they have no more reserves to draw on. While in a successful Confederate campaign, by 1865 you should be facing a vast host of regular or oversized Union brigades manned by inexperienced green recruits because you've killed off the Union veterans of the early war period, but in a panic, Lincoln has pulled out all the stops on conscription and called up 100,000s more of green draftees.
But it's easy to get overwhelmed with all the non-combat army management features, so they also have a kind of "skirmish" option where they let you play through the main historical battles with the fixed, historical orders of battle for the opposing armies instead of your custom campaign army and the AI's custom campaign army. But for sure, the replay value in the game is to be found in the campaign mode.
The single area where UG: Civil War falls short of UG: Gettysburg is in some of the terrain art assets for the Gettysburg map in particular. In UG: Gettsyburg, they had a lot of custom art assets to accurately portray the various distinctive battlefield landmarks like the Seminary campus, the Cemetery, Gettysburg town, the railroad cutting, etc, which were replaced in UG: Civil War by historically-appropriate but generic art assets not specific to the actual buildings and landmarks at Gettysburg. So instead of the distinctive Seminary building with the rotunda, they just have a generic 1860s brick mansion kind of building, instead of headstones at the Cemetery Hill, they just have boulders, etc.
