On the last day of May, we convened a roundtable of Fools to determine The World's Best Biotechs. But we were left asking ourselves: What about the other side of the coin? What are some biotech names potentially not worth the value investors are putting on them? Our assembled experts aren't pulling any punches, no matter how well liked some of these stocks are.

Fool contributor Rich Smith: My nomination for the biggest risk in biotech? No question: It's MannKind (Nasdaq: MNKD).

Oh, I know it's a popular stock. And I admit to some admiration for the company's efforts to make diabetes treatment easier and more user-friendly with its Afrezza inhalable insulin product. I just don't think the stock is a particularly good investment.

Why not? Let me count the ways. To begin with, MannKind is a company without a product -- or at least not one it's currently selling. Over the past 12 months, MannKind has recorded all of $143,000 in revenues -- not a lot for a $500 million company. Lacking revenues, MannKind also has no profits, and analysts see no hope that profits will appear either this year or the next.

Meanwhile, the company continues to burn cash that it doesn't have. Negative free cash flow for the past year topped $150 million. The $16 million penalty MannKind agreed to pay Merck (NYSE: MRK) last month won't help matters, either. As MannKind continues shoveling cash into its perpetual cash furnace, I expect that it will either dilute existing shareholders with new share issuances or add to its already crushing burden of $386 million in net debt.

Fool contributor Sean Williams: There is no bigger risk in biotech than betting the boat on a nonexistent pipeline -- and that's exactly what you get by investing your hard-earned money in Cell Therapeutics (Nasdaq: CTIC).

Cell Therapeutics holds the distinction of burning through $1.5 billion worth of shareholder money since its inception and has very little to show for it. Its primary drug candidate, pixantrone, which it hopes can be used to treat various types of cancer, recently failed to impress the FDA, leaving its prospects in limbo. Although the drug wasn't fully rejected, its prospects look bleak, considering that Cell Therapeutics wasn't able to meet its own clinical enrollment guidelines.

The company also has Opaxio, an experimental drug aimed at treating ovarian cancer, in phase 3 trials, but let's remember that Opaxio didn't get past the FDA when it was brought before the agency to treat non-small-cell lung cancer. Bearing a history of failure, it comes as a slap in the face that the company sold another 17.6 million shares just two weeks ago, diluting shareholders even further. With a Swiss-cheese pipeline and a ballooning shareholder deficit, this is a risk you can't afford to take. 

Fool contributor Brian Orelli: There are plenty of bad biotechs out there that investors should avoid, but the ones with solid data can be just as dangerous. Consider Pharmasset (Nasdaq: VRUS), which has a market cap of $4.6 billion despite not having any drugs in phase 3 development. Sure, the drugs -- two phase 2 compounds and one phase 1 molecule -- treat hepatitis C, a very large market, and the data that has been presented looks good. The company even convinced Johnson & Johnson (NYSE: JNJ) to team up.

But it's hard to recommend buying at these levels; the risk just doesn't seem to justify the upside from here. By comparison, Vertex Pharmaceuticals (Nasdaq: VRTX), valued at $10.5 billion, has a hepatitis C drug already on the market, a cystic fibrosis drug that's on the verge of approval, and four other phase 2 drugs. Pharmasset could get there, and investors would be rewarded with a solid double from here, but it'll take years, and there's a risk that one or more of its drugs will fail along the way. This is biotech, after all.

David Williamson: Well, there you have it, Fools. You've heard from our experts, but now we want to hear from you. Sound off in the comments section below and let us know why we're either brilliant or idiots. And be sure to add Mannkind, Pharmasset, and Cell Therapeutics to Your Watchlist and never miss out on our latest coverage.