Blank-check companies are booming again—in the courts.

Two law professors are aiming at high-profile targets, filing suits that question the legality of a specific type of special-purpose acquisition company. Among them are billionaire William Ackman’s $4 billion Pershing Square Tontine Holdings Ltd. , Go Acquisition Corp , co-founded by veteran entrepreneur Noam Gottesman, and E.Merge Technology Acquisition Corp.

Another line of legal attack is more typical, focusing on SPACs whose shares suffer big falls. These are targeting once-hot companies like electric truck maker Nikola Corp. , which class-action suits claim defrauded investors.

So far this year, there have been 19 of these more typical class-action lawsuits concerning SPACs filed in federal court. Five were filed in all of 2020, according to insurance brokerage Woodruff Sawyer & Co. In 2019, before the SPAC boom lit up the markets, there were two suits.

SPACs were a more popular target for federal class-action lawsuits in the first half of this year than other hot-button litigation areas, such as Covid-19 or cryptocurrencies, according to consulting firm Cornerstone Research.

This sharp uptick is likely just the beginning, insurance brokers and lawyers say.

Private companies flooded to special-purpose acquisition companies, or SPACs, to bypass the traditional IPO process and gain a public listing. WSJ explains why some critics said investing in these so-called blank-check companies isn’t worth the risk. Illustration: Zoë Soriano/WSJ The Wall Street Journal Interactive Edition

SPACs are shell companies that raise money and list on an exchange, usually with the goal of merging with a private firm and taking it public. As a suddenly hot alternative to initial public offerings, they raised more than $200 billion since the start of last year, according to data provider Dealogic.

As deals made by these SPACs begin to struggle, lawsuits are rising and regulators are stepping up scrutiny of the market. “I suspect there is going to be a lot more litigation to come,” said Kevin LaCroix, executive vice president at brokerage RT Specialty LLC, who writes a popular blog on insurance that protects company officers against litigation, including SPAC lawsuits.

Mr. Ackman has sought to rally Wall Street against the litigators. In a letter to shareholders last week, revealing his SPAC may now return their funds, he said the “basic issues raised [in the lawsuit] apply to every SPAC.”

The three lawsuits filed last week by law professors Robert Jackson, a former SEC commissioner and New York University professor, and John Morley, who teaches at Yale, are all based on the same legal theory. The lawsuits allege that the SPACs are operating in breach of the Investment Company Act of 1940, which governs companies whose primary business is investing in securities. The act imposes tougher constraints on issues such as compensation than the Securities and Exchange Commission rules that typically apply to SPACs. Insiders at these blank-check companies can make millions, even if the share price struggles, The Wall Street Journal has previously reported.

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Mr. Ackman said in the letter to shareholders that “while we believe the lawsuit is meritless, the nature of the suit and our legal system make it unlikely that it can be resolved in the short term.” A spokeswoman for Go Acquisition declined to comment. Guy Gecht, the co-chief executive of E.Merge Technology, declined to comment.

Go Acquisition and E.Merge Technology each awarded their founders special shares giving them a stake equal to at least a fifth of the outstanding equity, the lawsuit against the SPAC states. The shares, purchased for just $25,000, could be worth more than $100 million, according to the lawsuit.

Mr. Ackman said in his letter that the lawsuit against Pershing “may have a chilling effect on the ability of other SPACs to consummate merger transactions or to engage in IPOs until the litigation is resolved.”

Messrs. Jackson and Morley, the law professors who filed the suit, dispute this interpretation. Their lawsuits are focused on SPACs that have invested in treasuries and money-market funds for unusually long periods—typically more than a year—according to someone close to the litigation. The Ackman vehicle is also being targeted because of its earlier proposal to buy a 10% stake in Universal Music, rather than acquire a private firm, the professors said.

The professors said in a joint statement that their litigation is based on extensive research into the laws governing investment managers. “These suits seek to enforce the transparency and investor protections required by Congress in the Investment Company Act,” the statement added.

Most of the SPAC lawsuits to date have been “stock drop” cases, where investors allege they were misled by statements made by the company. Shares of many once-hot SPAC targets have fallen by half or more this year.

Insurance brokers say the plaintiffs’ lawyers in these cases are homing in on SPACs using a familiar playbook.

“Businesses going public through mergers with SPACs tend to be smaller and potentially riskier than those going public through IPOs. Sometimes those risks materialize and then there’s a stock price drop and then a lawsuit,” said Sasha Aganin, a senior vice president at Cornerstone Research. The surge in cases is a feature of these small-company risks, rather than anything specific to SPACs as a class, he added.

There is competition to bring some of the high-profile cases. Plaintiff lawyers are sparring in court over the right to lead a proposed class action lawsuit alleging the electric truck maker Nikola defrauded investors.

Nikola went public last year after merging with a SPAC and was at one point worth more than Ford Motor Co. But its stock has struggled since questions about its technology and products surfaced last year. Prosecutors last month indicted Trevor Milton, Nikola’s founder, on securities-fraud charges, saying he “lied [to investors] about nearly every aspect of his business.” Mr. Milton, whose lawyers have said he has been wrongfully accused, has pleaded not guilty.

Trevor Milton, CEO of Nikola, was indicated on securities-fraud charges in July. His company went public after merging with a SPAC.

Photo: massimo pinca/Reuters

The risks of lawsuits against companies going public via SPACs are seen as greater than for traditional IPOs, judging by the premiums charged for directors and officers insurance that protects executives against such litigation.

This directors and officers coverage for a company that is newly merged with a SPAC is “often 20% to 30% more compared to a similar company going public through an IPO,” according to Priya Huskins, a senior vice president at brokerage Woodruff Sawyer.

“The sense we have from the insurance underwriting community is the concern that companies going public through a [SPAC merger] are less ready” for the additional rules and scrutiny that come with trading publicly, Ms. Huskins added.

Write to Jean Eaglesham at jean.eaglesham@wsj.com