Here are a few thoughts from my limited experience with the design.
The design is simple and elegant. There are fewer pieces to cut and fewer seams to sew or fail.
The design limits your ability to pick material for specific purposes. For instance, texture of the front panel against your back, strength of the material at the shoulder strap attachment points, and abrasion resistance on the bottom panel. I don't think this is much of an issue unless you have those specific needs.
Because there is no bottom seam, the bottom edge of the back pocket must be sewn to the face of the back panel. This isn't really a problem, and it can protect the bottom edge of the pocket from abrasion when you set your pack down.
The design limits your ability to shape the bottom panel. It's a fixed rectangle. Ninety degree corners are hard to sew, so you may want to round the corners of the side panels instead. This in turn can move the bottom shoulder strap mounting points up a bit.
Because the seams will be so long and they negotiate corners, it's more important to use pins or alignment marks to avoid fabric 'creep'.
Here is an illustration showing rounded corners on the side panels, bottom seam of back pocket, and lower shoulder strap attachment points:
Hope this is helpful,