The Universal Century mode lets the player play through the events of the TV series Mobile Suit Gundam and Zeta Gundam. The player can even play as characters from the villains' sides. In fact, just about every named Mobile Suit pilot from both TV series is playable in the Universal Century mode, from major characters like Amuro Ray and Char to relatively minor characters who only appear in a few episodes. Sadly, the weakness of the Universal Century mode comes from this large cast.
In the Universal Century mode, every character follows a specific route through the game based on the major battles that character fought in the two TV series. Of course, there is a lot of overlap in these stages, since most major battles in Zeta Gundam or Mobile Suit Gundam involved anywhere from two to almost a dozen named pilots. So, many different characters will be expected to fight the same battle on their respective paths. While the player has the chance the fight the same battle from both sides, he does have to fight the same battle over and over again.
The problem of repetition is compounded by the game's system of route splits and alternate history stages. A big part of the Universal Century mode is about changing history. The game lets the player win certain battles with a character who died in that same battle in the original series. If the player does this, it unlocks an alternate history route for that character, and possible for other characters. For example, Jarid has two points where his path branches out into an alternate history path if another specific character survives. Unfortunately, while this system gives the player the satisfaction of changing history, it adds a lot more repetition to the game.
As an example of this problem, let's look at Jarid's path. Jarid has two places where his route splits: the battle where Lila dies, and the battle where Mouar dies. If Lila manages to survive her fateful battle, Jarid's route branches, and a new chain of missions becomes unavailable. The problem is that the new unlocked missions are identical to his original set of missions. The only variation is that Lila is his new teammate (and CPU teammates are not typically a major factor in a mission). If Mouar survives her battle, the same thing happens. Furthermore, the new paths that are unlocked for Mouar and Lila assume they fight alongside Jarid. So, they also go through Jarid's remaining missions, and are even given the same mechs Jarid gets. So the player is expected to fight through the exact same set of missions five times with just three characters.
The problem comes up without the alternate history element, because some very minor characters have been made playable. For example, Yazan, Dungel, and Ramsus are a team of three pilots who always fight together in Zeta Gundam. Yet, they each have individual paths in Gundam vs. Zeta Gundam. All three characters fight virtually the same missions, using the exact same set of mechs. It is the same as being forced to play the exact same part of the game three times.
The problem of all this repetition could have been mitigated somewhat if the game developers varied the wining conditions and opponents between different paths more. By varying the winning conditions from "defeat the Zeta Gundam" to "hold out for two minutes", for example, fighting the same battle again could have been more interesting. Even better, the developers could have embraced the alternate history element of the plot more heavily, and changed the mechs the player faced around on the alternate history paths. It would have been fun to fight against Kamille piloting the Super Gundam or ZZ Gundam instead of the Zeta Gundam on some alternate paths. Or maybe fight Quattro piloting the Jiong instead of the Hyaku Shiki.