They're related, but not interdependent. A game can make it very easy to hold a drift, but at the same time make it difficult to keep from drifting in the first place. Given that drifting is slower than grip driving, it would therefore be difficult to drive fast, but relatively easy to keep your car on the track and pointed in the right direction.ease of control does relate to how easy it is to drive fast, every game is easy if you don't drive at the limit. driving at 80% in any game and real life is easy.
Hell, the entire concept of most arcade racers is that it's easy and approachable for all players, but difficult to master and be the best. That kinda illustrates what I'm getting at: FM4 is one of the most hardcore console sims around, but it still has arcade-like qualities. Its unflappable handling is merely a transparent example of one of those qualities.
I'd say that's one point in Gran Turismo 5's favor -- it's not any more realistic (less so, in my opinion), but at least its cars can bite you. The mid- and rear-engined cars demand some respect, as they should. In FM4 I can drift the Shelby Cobra, Lotus Exige, RUFs, and even LeMans prototypes all day long without much concern. I'm pretty well-practiced at that sort of thing, but that only means I can recognize that it's just too simple.
To be honest, it's good that I can be this nitpicky. It shows how far the Forza series has come. 👍