I'm frustrated by ProtonMail's keyboard shortcuts, because they miss the point.
I suspect Proton added keyboard shortcuts for feature parity with the competition. They have shortcuts for most common commands, yet they fail to improve the app's accessibility. Why?
In GMail, the mouse is optional. They modeled the full state graph of the app, and they created keyboard shortcuts for most / all of the actions and transitions you can do in every state. That means you can navigate from any state to any other, and get everything done from the keyboard.
Proton screws this up, especially keyboard focus and chaining commands. For instance, if I type "gi" to go to the inbox, I then have to click to focus the message list before I can use keyboard navigation to select one of the messages there. At best I can do an action or two at a time before I break out the mouse. But the only reason I use keyboard shortcuts is to save my wrists from mousing!