OK. With sous vide you can have beautifully cooked, medium rare beef by roast beef by simply cooking it at 54.4C/130F. For a low work muscle, like a fillet/tenderloin steak, which is already tender then all you have to do is cook for long enough for the core of the meat to reach equilibrium with the water bath. Easy.
For tough meats, which are those muscles on the beastie that have done a lot of work during their life, the same temperature is fine so that you get that luscious medium rare. However, to give the connective tissues time to break down requires a longer time at that temperature.
For instance, I love beef cheeks. They're a flavourful, hard working muscle. Let's face it, there's not often in its life than a bovine isn't chewing. So, in normal cooking circles it would usually be braised so that the collagen in the meat has time to breakdown and give that lush texture.
I cook my beef cheeks at 54.4C/130F for that beautiful peachy pink blush. 2-4 hours is not going to be enough time to give the fibres and connective tissues time to break down. With some experimenting I've discovered that 60 hours will give me that beautiful buttery texture of the most amazing prime fillet steak. 72 hours will give me that luscious cut-it-with-a-fork texture of a traditional braise. But because it's all done at 54.4C it's all still got that gorgeous pink blush.
Mind you, this is an extreme example due to the cut of meat and the temperature at which I cook it.