There are some brilliant ideas in this tool, but sadly, it falls short of expectations when it comes to practical use. And there are a lot of similar add-ons or standalone apps that easily outdo it.
Let me start by markup tools. It allows annotation but it does not have a tool to create a circle or a tool that creates an arrow. One has to create them with a free drawing tool. In terms of the text, the font it uses is a serif font! It is unfriendly to the English writers, and outright unsuitable to Arabic, Persian or Hebrew writers. It does not support proper bidirectional text either.
In the meantime, Firefox already features a similar screen capture tool.