I suspect I'm in the minority but the biggest draw of Inbox for me is the UI. I have used gmail with Stylish rules to eliminate most of the default UI, but it breaks periodically with CSS changes (not to mention Google banned Stylish recently, I have yet to look into alternatives).
IMO the Inbox UI is miles ahead of default gmail, and all of the other graphical email clients I've found on web and desktop mimic the traditional email UI.
It seems like I'm left with changing gmail's CSS or using a terminal email client, but I'm not relishing it as it seems like a step backward when all I'd like is a simple webmail client with a nice UI.