Roku shares declined 10 percent Tuesday following the report.
Helfstein started his Roku price target at $28, representing 34 percent downside to Monday’s close.
The analyst noted the company is trading at 18 times fiscal 2018 estimated gross profit versus the 4 times average of its technology industry peers.
It is “difficult to justify this valuation, even with secular growth trajectory and market position,” he wrote.
Roku shares have been extremely volatile since its IPO. The stock rallied nearly 70 percent on its first day of trading and then declined more than 20 percent in less than a month.
The roller-coaster ride continued as the company’s stock rose more than 120 percent in the three trading days after it reported better-than-expected earnings Wednesday.
Roku declined to comment for this story.
— CNBC’s Michael Bloom contributed to this story.