With Notion, some helpful shortcuts exist. That said, simple things like switching inbox views still requires mouse usage or opening up their command centre via CMD + K and writing where you want to go, causing a bit of friction.
And while Notion's snoozing and send later features were more user friendly than Gmail's, they still required mouse usage to get the most out of the features.
If you want to reply in line, the process is that you need to copy the entire email, format it, and then use their quoting feature and then write your reply. This is tedious.
Notion Mail's snippets are enabled by default and they even give you some pre-set options with placeholders. You can easily access them by hitting / and selecting your snippet which is super handy. We also liked their placeholder functionality - its super easy to use as it guides you through each part that needs to be filled out with just hitting tab.
Notion Mail offers super basic offline mode but on their desktop app only and its limited to reading and composing emails only.