Winning goes hand-in-hand with popularity. It's true in sports. And it's true in investing.

You probably won't be surprised, therefore, that many of the most popular stocks on the Robinhood trading platform have delivered impressive gains over the last few months. That doesn't necessarily mean that their sizzling momentum will continue, though. Here are three Robinhood stocks that have more than doubled so far in 2021 -- and a look at whether they're still good picks to buy now.

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Aphria

Aphria (NASDAQ:APHA) ranks as the second-highest marijuana stock in the top 100 most widely held stocks for Robinhood investors. It trails behind only Sundial Growers. But while Sundial's huge gains have dwindled enough to put the stock up by "only" 90% or so in 2021, Aphria's shares have soared 106% year to date.

The biggest factor driving investors' interest in Aphria is the company's plans to merge with Tilray (NASDAQ:TLRY). This deal will make the combined entity, which will assume the Tilray name, the largest cannabis company in the world. 

Another key reason for Aphria's ascent, though, is the increased probability that major cannabis reform will be enacted in the U.S. Aphria can't jump into the U.S. cannabis market and hold onto its Nasdaq listing as long as marijuana remains illegal at the federal level. However, the prospects of marijuana decriminalization could set the stage for Aphria and other Canadian cannabis producers to expand into the lucrative U.S. market.

Bionano Genomics

There's a genomics revolution underway right now. Surprisingly, though, only one pure-play genomics stock is in the top 100 most popular list on Robinhood. That lone ranger is Bionano Genomics (NASDAQ:BNGO). The stock has been a major winner so far this year, with Bionano's shares vaulting 108% higher.

Bionano had generated even bigger gains earlier in 2021. By mid-February, the stock had more than quintupled year to date. However, a shift away from high-flying growth stocks caused Bionano's share price to tumble over the last couple of months.

Many investors remain enthusiastic about the company's prospects. Bionano beat analysts' Q4 revenue expectations. It expects to soon win accreditation in select European markets for Saphyr-based laboratory-developed tests for acute lymphocytic leukemia (ALL) and facioscapulohumeral muscular dystrophy (FSHD). The genomics testing company also thinks that it will have 150 Saphyr gene-sequencing systems installed by the end of this year, which would represent a 50% increase year over year. 

Ocugen

Ocugen (NASDAQ:OCGN) stands out as one of the biggest winners among the top 100 Robinhood stocks. Shares of the biotech have skyrocketed 277% year to date. At one point this year, Ocugen was up more than 750%.

Until late 2020, Ocugen's primary focus was its gene therapy candidates targeting eye disease. However, in late December the company announced a partnership with Bharat Biotech to co-develop COVID-19 vaccine Covaxin for the U.S. market. 

Covaxin has already won authorization in India. Bharat reported results from a late-stage study conducted in the country that demonstrated an 81% efficacy rate. Ocugen stands to receive 45% of any profits if the vaccine is authorized or approved for sale in the U.S. The company is working with U.S. regulatory authorities to develop a path for Emergency Use Authorization (EUA) for Covaxin.

Are they buys now?

There are reasons to like all three of these Robinhood stocks. However, there are also some reasons to stay away from all three.

For example, Aphria's disappointing Q4 update earlier this week underscored the challenges the company faces. The COVID-19 pandemic especially impacted its financial performance. But pricing pressures in the Canadian cannabis market are also hurting Aphria.

Bionano arguably has the most solid growth prospects of the three companies mentioned. There's one problem that I see, though. Bionano estimates that its total addressable market is between $2.6 billion and $3.8 billion. The company's market cap already stands close to $1.8 billion. Much of Bionano's potential growth is already baked into its share price.

What about Ocugen? It's possible that Ocugen could be too late to the party to become a big winner in the U.S. given there are other vaccines already on the market with higher efficacy levels than Covaxin.

Maybe Aphria, Bionano Genomics, and/or Ocugen will deliver even greater gains in the remaining months of 2021 than they have in the first part of the year. My view, however, is that there are stocks that offer better risk-reward propositions than these three popular Robinhood stocks.

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.