Well the favored soul mechanically isn't as bad as the really bad stuff (like the cleric and wizards which are both stronger) though it is still very breakable.
In the balance difference is not as wide. In fact I would not mark any one class as "broken" in a 3e sense. Classes like wizard, fighter, warlord, and ranger are clearly top of their respective class roles (in some cases by far) but they do not break the game like a 3e wizard could (4e eliminated many of the alternatives to damage in ending encounters). In addition even the weakest classes actually can do enough that they can contribute to standard encounters with basic optimization (such as taking basic feats like expertise) as expected though of course other classes could easily do more. This is in contrast with many weaker 3e classes which if you go by the tier system are defined as classes that cannot adequately contribute to a standard encounter.
If you want to use the 3.5 tier system and apply it to 4e in general you would find that the classes would end up high tier 3 (close to broken but not quite) to high tier 5 (class isn't that good but is just good enough that it can contribute with a basic solid build).