Sure, functionality-wise Ubuntu and also other Linux operating systems can definitely match the palette that OS X offers. The problems with Ubuntu are starting if you are getting into the area of peripherals:

Drivers for some things are missing or are not performing as comparable Hardware does under OS X, it gets even worse when you're struggling with HiDPI: Have you ever tried to have one non-HiDPI and one HiDPI screen as when using your QHD laptop screen together with a FullHD secondary monitor? Total pain in the ass under Ubuntu and in contrast very easy on OS X.