The UX is nowhere near close to native apps, particularly on platforms that take UX seriously like macOS, iOS or Gnome.
People often overlook some of the functional UX the web brings to the table. For example: on the web I get consistent search and text highlighting behavior, consistent notifications, consistent navigation behavior (back buttons, history), I can tab to focus (usually), and I can use plugins to customize the content. Even the idea that you can reset the application state with a refresh is something I wish I had on some native apps.
Some widget will be rendered wrong, they'll use the wrong font (or even hinting!), right click may not work as expected, they won't use multiple windows correctly, etc.
Even React Native gets some of this wrong, although few use that on desktop.
If you're a user that appreciates consistency on your OS, all of this will stick out and annoy you. And you'll end up using Apple's Mail.app over GMail's UI.