Kodi is just a media player. It's a very sexy media player, but it's still just a media player.
A lot of Kodi Boxes are ROKU boxes, Roku is actually the software - it's the difference between Mac and Windows.
I run Kodi on a Raspberry Pi, but I've never used Roku, so I can't really tell you exactly what the differences are, but I can tell you there are a quintabajillion addons for Kodi. If you're looking for something, it's got it (but not all in the official repos). The only thing it hasn't got is 4OD, but 4OD is the single biggest PITA on the planet, so that's to be expected.