I don't use the native app or its complication on the watch, but have you tried Outlook as a mail app? It's decent on the watch IMO, though not sure what you'd need on the complication.
The easiest way to see the difference between any 3rd party mail app and the Apple mail app is when you receive a notification on the iPhone. Press your finger on the notification and let it open a window to see the email. Most 3rd party apps will only display a small portion, in pure plain text of the email, while the Apple mail will display the full email exactly how it looks in the app, with all its graphics.
This holds true on the Apple Watch too. 3rd party app notifications only show a small portion of text while the Apple mail notification will display the entire email, graphics included, when opened.
Being able to see all this info via the notification itself, saves me from having to go into the app itself many times, to be able to know more about the email.