Roku (NASDAQ:ROKU) bashers never learn. Short interest on the streaming video pioneer rose to 9.3 million at the end of August, the largest numbers of shares betting against Roku stock since February. It hasn't worked out well for the naysayers. Roku stock has risen more than 20% so far in September. 

The stock is now a week away from completing its first year on the market, and it's been one of the market's biggest winners. Roku has popped more than fivefold since going public at $14 in late September of last year. It's been volatile along the way, but do you really want to short a stock when momentum -- and, just as importantly, the fundamentals -- keeps improving? It's a dangerous art.

Roku's operating system running on a TCL smart television.

Image source: Roku.

 There's always something good on TV now

Bears will argue that the problem with Roku as an investment is that the stock has outrun the fundamentals. Revenue hasn't made a fivefold advance, and certainly the number of Roku users hasn't quintupled since last fall's IPO. The rub here is that boiling down the boo-bird arguments to how valuation multiples have expanded isn't the way the market works. 

Roku is a five-bagger, while quarterly revenue has only risen 57% over the past year, but expectations in general have changed. Top-line growth at Roku was clocking in at 22% in 2016 and 23% through the first half of 2017 ahead of the shares hitting the market, and now we have a company putting a little more weight on the accelerator pedal. 

The model being milked here has also changed dramatically in a short time. Roku was seen primarily as a hardware play a couple of years ago, trying to stand out with set-top boxes and sticks that would smarten up dumb TVs. Hardware has become a harder market to crack now that tech giants are practically giving away their gadgetry, but this has simply opened the door for Roku to thrive as a platform provider.

The 57% spike in revenue for Roku's blowout second quarter is the combination of a 24% gain in hardware that's padded by a 96% surge in platform revenue. Platform revenue overtook the actual players in terms of revenue mix earlier this year, and that's a great trend since that's where the higher margins can be found. Let's not even get started on the scintillating consumption and monetization trends that Wall Street bulls on the stock keep harping about.

If growth kicking things up a notch and the model being upgraded to the point that Roku is the service-agnostic platform of choice in the industry don't sway your bearish bent, let's just wrap this up by getting historical. The last time that short interest was this high was at the end of February, when there were 9.9 million shares sold short. The stock was just crossing $40 at the time. The shares have gone on to nearly double since then, so the value of the short positions is a lot higher now than it was then. Things didn't work out too well for the Roku doomsayers, and it won't be a surprise if things don't play out as planned for them this time around either.