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I disagree with mobile first design in general. Some apps are meant to be desktop apps, and you can't make them into mobile apps without either severely dumbing them down or just creating a different apps.

Like outlook, which is also an email client. I'm sure there's an outlook smartphone app, haven't used it, but it's certainly a completely different UI and application.

I can usually spot mobile first design from a mile away. Because the app looks stupid on desktop and sort of feels like a toy. Huge buttons, enormous white space, menus for no reason.