I agree with this critique. A few additional comments: your boxes looked great! However, some of your exercises had a lot of empty space where you could have added more. The plotted perspective is the most obvious example of this. Make sure you're filling the page to get the most out of the exercise. You did a much better job with the organic perspective exercise filling-the-box wise. Similarly, in your line-drawing exercises and your plane exercises, most of the lines were the same length. Work on challenging yourself by varying up the lines. It's great if you can draw one length of line really well, but how do you do with tiny lines? Or really long lines that cover most of the page? Look for ways to vary it up that way you are sharpening multiple skills at once.

Best of luck with the box challenge.