Things are going so well for Roku (NASDAQ:ROKU) these days that even a prominent naysayer is now long the streaming platform operator. Roku stock rose 9.7% higher last week, taking off after Citron Research's Andrew Left announced that he is now an investor.

Left became a vocal short in late November, arguing that the stock was a joke in a fiery tweet. He would go on to appear on CNBC, trading jabs with Needham analyst Laura Martin who was bullish on Roku with a $50 price target. Left is now a believer, and with the stock still trading lower than where it was his initial bearish tweet he was right the first time around. It remains to be seen if he'll be right now as a bull and voicing at least one component of Needham's original bullish thesis.

Roku on a Windows 10 platform.

Image source: Roku.

 A change of heart 

Citron's Left argues that Roku is trading at the largest discount ever to its over-the-top peers, largely the product of Netflix's (NASDAQ:NFLX) monster appreciation in recent months. Left points out that Roku is the only pure play in this niche that generates ad revenue, something that Needham's Martin argued late last year. Her bullish case played up Netflix's ad-free platform as a reason for advertisers flocking to Roku to get their messages across. 

Left also argues that Roku could be a Netflix acquisition target, has an impressive list of top shareholders, commands four times the short interest as Netflix, and is toiling away in a market that has been validated by cord-cutting consumers. 

"I want to be long," Left concludes in reversing his initial bearish stance from six months earlier.  

Roku's latest well-received quarterly report shows a company transforming from a seller of hardware devices into a fast-growing platform. Revenue rose 36% in the first quarter, but it's actually the combination of a 3% decline in player revenue being more than offset by a 106% surge in platform revenue. There are now 20.8 million active accounts leaning on Roku to fuel the growing streaming content that is available, 47% more than the audience that Roku's platform was commanding a year earlier. Average revenue per user has soared 50% over the past year. 

Losses are narrowing. Roku is jacking its full-year guidance higher. It's easy to see why Left has decided that recommending a short position in Roku is dangerous, especially with the stock falling behind in matching Netflix's scintillating gains in recent months.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.