After a few weeks with the game, as a complete beginner in guitar, I would like to share my thoughts about the "Rocksmith method".
First, I think it is very difficult to make a game and a learning method at the same time.
The traditional way of learning is repetition, over and over again, until mastery. Quite the opposite of fun.
By enabling you to play along immediately, RS is great for the motivation and let everyone enjoy the experience.
As I see it, in order to let the game be fun, RS is based on many small steps and as difficulty rise slowly, you don't have big hurdles.
But, if you have never touched a guitar before, I think you may miss some things by playing only RS.
For example, on the easiest level, you can't understand the logic behind the notes you have to play. Sometimes, arpeggios are based on a chord, and unless you identify it by yourself, you may struggle to play the notes individually by moving your left hand fingers instead of using the chords position...
The games doesn't make you practice the stuff needed to play a song beforehand. Once you are doing well with single notes, the game throw you chords you have never seen and you have to train _after_ failing to play correctly, which can be quite frustrating.
Another point is, until you can master the song, notes are changing almost every times (since you are making progress). Therefore, you can't play anything by yourself.
Trying to remember a song until 100% mastery notes is pointless due to the fact that you may play the wrong way (for example, left hand positioning based on the chords).
I think this is the main reason why you can't fix the difficulty level for a song. It is meant to play fully, before that, it is just training.
The fact you can't play by yourself or be able to show what you can do (tried once after getting a good level on single notes and failed when never seen chords appeared) may be hard for beginner.
Finally, bad habits is the worst thing when you are trying to learn anything. By challenging you all the time, RS doesn't let you the time to make things correctly and I fear you may end with bad habits that may take some times to correct later. For example, when playing scales or chords, at the beginning, it is important to have a good hand positioning and be careful with your fingers in order to do it well. Practicing speed without correct position may be detrimental in the long term.
For me, RS is a great way to discover the instrument by exploring different types of music and sounds. By letting you play along from day 1, it is a wonderful way to make you want to play. But, unless you know already the instrument, I think you should complement it with real practice if you want to be a "real" guitar player.
A friend of mine showed me JustinGuitar, but there are also a lot of other resources online. A "real life" teacher may also be a good idea to point about bad habits early.