I love these value studies! You captured the shadows really well.
For the portraits, one critique I have is to study the references more than doing style exercises. With the exaggerated portraits there are a few fundamental things that are off, like a mix up with the angle of the eyes versus the jaw or how far apart the eyes are. I think if you really study the photos and features of the face you can enhance you stylized drawings.
Sorry for the chicken scratch line art
With this one I think you did a nice job with the placement of the facial features. They eyes, nose and mouth have a good amount of space between them.
One main issue I found is the eye line and main line for the eye were straight, but your reference, the eyes you drew and the jaw are all tilted. This created a disconnect between the top and bottom halves of the face and made the jaw look really big.
Also, the skull/face shape weren't as defined as they could have been. For your stylized portraits you're doing detailed constructions and adding realistic elements to the shape of the nose and mouth, but the eyes, chin and skull are more simple.
I think if you do more realistic studies and bring what you discover into your stylized drawings you can push your work further.
Hope this helps!