In addition to selling its own streaming hardware and licensing its technology to TV manufacturers, Roku makes money by selling ads when customers watch certain channels and shows from the service. The Roku Channel lets users watch hundreds of free ad-supported shows and movies and subscribe to premium channels like Showtime and Starz.
But Disney, AT&T's WarnerMedia and Comcast's NBCUniversal are all planning their own streaming services, which "could limit the amount of library content made available to Roku Channel over time," Macquarie said. Disney has already made plans to pull its movies from Netflix so its own platform can stream them exclusively. And Apple is expected to announce its own service at an event March 25.
The analysts also expressed concern that Roku's business model could become outdated if the industry moves toward a more open approach to targeted TV ads. Disney, NBCU and CBS are among networks in a consortium called Project OAR (Open Addressable Ready), pushing an open approach to ad targeting that "may conflict with Roku's more closed system," Macquarie said.
According to Variety, Project OAR aims to launch in early 2020.
Still, the Macquarie analysts raised their price target on Roku from $57 to $66 and increased their revenue estimates for this year and next about by about 2 percent.
Disclosure: Comcast is the owner of NBCUniversal, parent company of CNBC and CNBC.com.
Subscribe to CNBC on YouTube.
Watch: Is now the time to buy Roku?