I wouldn't call ROKU a competitor of NFLX. They are actually more like partners.
ROKU mainly sells ROKU devices and licenses the ROKU OS for TVs. ROKU devices & TVs offer many different streaming channels. You can actually watch NFLX on a ROKU device. So ROKU & NFLX are synergistic.
And while ROKU does have a channel, it isn't in the same ballpark as NFLX.