I haven't really noticed any real progression of difficulty within each course from song to song, but I've always thought that, while they are challenging, the BGIAB I solos where not as advanced as I had hoped. I was hoping that they would be solos that I've heard Griff and others play at gigs.
In that sense there is a nice progression from I to II. If you can master the solos, fills and rhythm in I, you will be in a great position for them in II.
BGIAB II is more along the lines of what I was looking for. These are solos, fills, riffs and rhythms that I'll be really proud to play (assuming that I can ever really learn to play them well).
Again, more of a leap forward than any "progression".
As with any course some songs will be easier for some and harder for others depending on their musical background.