I was very disappointed in the Forge World homeworld description, in that it states over and over that Forge Worlds tithe to the Imperial Guard. This is simply not canonical.
The Treaty of Mars granted political autonomy to the Mechanicum of Mars and its Forge Worlds scattered across the galaxy as well as an exception to the Imperial Truth so that the Cult Mechanicus' adherents could continue to practice their faith.
Forge Worlds supply technology, weaponry, ships, and tech priests to the Imperium, in exchange for navigators and astropaths; and that is the extent of their obligation.
In regards to the DH character creation system, this could create some very interesting character customization. The big one being that the Imperial Guard background could be used to represent Skitarii characters, the militant arm of the Mechanicus.
For more on the Skitarii:
The Skitarii, also known as the Tech-Guard, are the cybernetic military forces of the Adeptus Mechanicus of the Imperium of Man. They are used to defend Mechanicus Forge Worlds, manufacturing installations elsewhere in the Imperium and often accompany high-ranking Tech-priests and Magi as their bodyguards. Skitarii also serve as the armed forces that defend the Mechanicus' Explorator vessels that explore the galactic frontier. Skitarii safeguard Titans on campaign from boarding actions and prevent hostile forces from reaching the war engines' dead zone, where its main weapon batteries are useless. A number of Titan Legions maintain their own Skitarii detachments for this purpose, and if they do not, their Mechanicus masters will provide ample forces to protect the God Machines. The term "Skitarii" is formally applied by the Tech-priests of the Mechanicus to all of the military forces under their command, including infantry and armoured vehicles, save for the Titan Legions and the Knights. However, the term colloquially has often referred solely to the Adeptus Mechanicus' cybernetic infantrymen.