Tesla has a market value of nearly $48 billion, making it one of the 150 most valuable companies in America. Yet Nomura Instinet analyst Romit Shah today put out research saying that Tesla is "no longer investable."

The reason: What Shah refers to as "the erratic behavior of Elon Musk."

Shah says that Tesla could be a good bet with a different leader, but chaos monkey CEOs aren't the only reason that securities are sometimes considered uninvestable.

• The S&P 500 won't include unprofitable companies, which is why Tesla isn't in the index.
• Memories are still raw when it comes to synthetic derivatives, which no one really understood and which therefore no one should have been investing in before the financial crisis.
• Similar arguments can be and often are made about cryptocurrencies, today.
• Environmental, social, and governance considerations can also make a stock uninvestable.
• Many investors won't touch gun manufacturers, for instance, or tobacco companies, or companies like Snap which give shareholders no voting rights.