I am a hard man to please. None of the screencasting / screen capture options have everything I need. I love the way +Screencastify handles uploading to YouTube and the choice to sync to Google Drive. I love the simplicity of TechSmith SnagIt (most of the time) and its all in one approach. The problem with simplcity is that there are times when more advanced features are needed. This is where the Awesome Screenshot App comes in.
While the app has the circle, square, and text options you find in TechSmith SnagIt, it expands on that with some advaced features that you do not need every day, but might for a special project. One of my favorite is the squiggly arrow, an arrow you twist and curve like the image above. Another personal favorite is the blur function which allows you to blur a part of an image or text. There are certainly other advanced features but those two are my favorites.
On a Chromebook you can open your file from Google Drive, annotate away, and then save it back to Google Drive under a new name. This all done offline and does not appear to sync with Diigo directly, which I prefer.
Overall, the big question of our (Chrome) age is when to use an extension and when to use an app. TechSmith answered this question by going app free, Awesome Screenshot answered this question by offering advanced editing and annotation functions in an offline app. Time will tell which approach is best.