They never look and feel exactly like native apps, even the ones that try a bit.
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.
