Star Wars: The Force Unleashed is a certified classic. It is a Star Wars game done in the style of a spectacle fighter, such as God of War and Devil May Cry, where the player gets to take the role of one of the most powerful Force wielders in the expanded universe canon. Vader's Secret Apprentice, aka Starkiller or Galen Marek, is a near-unstoppable killing machine intended to root out Rebel dissidents and the Jedi that remained after the Purge following Order 66.
However, the Star Wars expanded universe was always vast and complicated, and a lot of additional history was added between Revenge of the Sith and a New Hope. Naturally, things are going to conflict and seem inconsistent, especially since this game is technically a prequel to a movie that came out in 1977.
4 Why Did None Of This Get Even Hinted At In The Movies?
This is an easy nitpick for any after-the-fact expansion on an existing IP, and it's one that can be said for just about every sequel ever made -- let alone a sprawling expanded universe like the one that Star Wars has. That said, it does seem strange that Darth Vader's ultra-powerful secret Sith apprentice who is even discovered by Emperor Palpatine never comes up again in A New Hope, Empire Strikes Back, and especially in Return of the Jedi when Luke comes face-to-face with the Emperor.
It's not unfeasible that Starkiller would never be mentioned again; it just seems highly unlikely. In the game's defense, it seems that the only living individuals who knew about Starkiller were Darth Vader, Palpatine, Juno Eclipse, and Rahm Kota. Two of those individuals become Rebel fugitives who never show up in the movies, and the other two are Sith Lords with enough going on so that Starkiller may have genuinely slipped their minds by the time of A New Hope.
3 The Evolution Of The Stormtrooper Armor Is Inconsistent
Force Unleashed attempts to show a new kind of Stormtrooper armor that exists as a midway point between the Revenge of the Sith Clone Troopers and the original trilogy's Stormtroopers. By the end of the game, you even begin to see the traditional Stormtrooper armor in use. That said, the Revenge of the Sith armor already sort of looks like Stormtrooper armor, so this armor design seems a tad redundant. Many of the Stormtroopers encountered by Starkiller are technically Naval Commandos, EVO Troopers, and Jump Troopers -- but that doesn't change the fact that there are a ton of them in the game while never appearing in anything else taking place around this point in Star Wars history or afterward.
The problem is that even the pre-Disney expanded universe never really added to this idea. Games like Battlefront II never acknowledged an armor variation between the Clone Troopers and the original trilogy Stormtroopers. Post-Disney does this even less justice, showing some different intermediate armor variations in things like Jedi: Fallen Order and the Solo movie. The Bad Batch television series shows a gray-black Clone Trooper armor used by the first Stormtrooper recruits to differentiate them from the Clone Troopers.
2 The Curious Death Of Jedi Master Shaak Ti
Jedi Master Shaak Ti is something of an infamous Jedi for longtime fans. She received a spotlight from Gendy Tarakovsky's, as well as Dave Filoni's Clone Wars. There is a deleted scene from Revenge of the Sith that shows Shaak Ti being killed by General Grievous and another showing her being killed by Anakin Skywalker.
The Force Unleashed attempted to nullify both by showing her death at the hands of Starkiller on Felucia. After Disney erased the expanded universe, the final fate of Shaak Ti was in flux for a bit. However, Filoni's Clone Wars ultimately answered this with a Holocron showing Darth Vader killing Shaak Ti while she meditated.
1 The Sequel Is Really A Bit Of A Continuity Mess
While much of the first Force Unleashed could theoretically have taken place without being mentioned in the original trilogy movies, the sequel really kicks things to the next level in terms of improbability. Vader possibly clones Starkiller, but it's also implied that this Starkiller might have just survived the events of the first game. Galen Mark reunites with the Rebellion and launches an all-out siege on Kamino.
This attack on Kamino (which would be one of two Kamino revolts if one keeps in mind the Kamino mission from Battlefront II), ends with the Rebels actually capturing Darth Vader alive -- which would have been a big freaking deal that would have had to have been referenced in the original trilogy. That said, none of this is followed up on, as the Force Unleashed never received a third installment. The game's DLC actually ties into a Star Wars: Infinities story (think of Marvel's What If... but for Star Wars), ultimately implying the whole series is a Star Wars: Infinities concept.