Roku shares had their worst day in over a year Wednesday after analysts at Macquarie Research said the stock's 2019 rally isn't justified given the threat of competition from a host of larger media companies.

Roku plunged more than 14 percent to $60.74, the steepest drop since February 2018, and the third-biggest decline since the company went public in 2017.

Even after the slump, Roku shares have almost doubled this year. Analysts at Macquarie downgraded the stock Wednesday to neutral, saying it's at "a full valuation," and that the ad-supported video on demand (AVOD) space is becoming intensely competitive.