The XNA Game class only updates GameComponents that have been added to it's Components collection.
So if you have an ObjectManager with a list of GameComponents, and call the Update method on those GameComponents manually, that is fine, as long as they have not also been added to the Game class Components collection.
But creating an ObjectManager for maintaining a List of GameComponents would be a bad design because the GameComponents have some protected methods (like I think LoadContent), so your ObjectManager class could not access these without redirection.
So if you want to have an ObjectManager class, it would be better if you didn't use GameComponents, and just roll your own. If you roll your own, you could have them inherit from the same interfaces that GameComponents do, if you want to maintain some sort of commonality.