The National Labor Relations Board failed to conduct the legally required economic analyses on the potential impacts of its “joint employer” proposal on small businesses and labor unions, according to a group of academics.

Although federal labor law prohibits the NLRB from hiring economists, the board ignored existing research and other evidence to estimate the costs of its plan or assess the ramifications of keeping its current standard, professors of labor law, antitrust law, and labor economics said in comments filed Jan. 14.

The academics’ comments highlight potential legal landmines the NLRB may have to navigate in court if the...