The Imperial German Bodyguard are a foreign elite within the Western Roman army. They are well armed, well trained and brutally effective.
The "germani custodes corporis" are loyal to the ruling family because they must be; their continued survival depends upon it. As barbarian foreigners they have few, if any, friends within the Roman army or political establishment. They can be trusted by the Imperial family because of this one fact. If the Emperor dies then they would soon be put to death by the Roman mob. As a result, the bodyguards are unlikely to turn against their masters.
In this they are the exact oposite of the old, disbanded Praetorians who had a nasty tendency to involve themselves in imperial politics and the imperial succession usually at the point of a sword and often for money.