"I've never found Linux all that usable!"
You log in, like on Mac and windows. You install updates when prompted or turn on auto install like on Mac and windows. You browse files in a file manager like in Mac and windows. Plug in a USB stick just taken out of the packaging and drag and drop files to and from it, just like in Mac and windows. And just like in Mac and windows you are supposed to unmount hat stick before removing but just like in windows you probably will just risk pulling it anyway.
Everything you need to do to a file on the Linux desktop can be done with right clicks or left clicks like on windows, in fact the right clicks can be customised if wanted.
Browse the web? Who isnt using/has used Chrome or Firefox. Well both are on the Linux desktop.
Upload files to any cloud service that uses HTML5 in these browsers? Drag and drop.
Access emails? Easy, but if you really need to use Office 365, you can do so in your browser.
The stuff you listed like certain CAD and photo editing software are specific cases where somebody needs (wants) a bit of software they are used to. This is not a usability issue at all as many of those people would suffer similar issues when switching to using a Mac. At the end of the day its not an argument about the usability of an OS or its desktop but an argument of the specific software supporting your desktop.
Would you think I could say the Xbox One is borked and not usable because I cant play mario kart or Zelda on it?