The shadow economy is beneficial in the way that the money people make from their "secret" jobs is used for personal consumption which rises the economy. It also allows people that are having trouble getting a full or part time job in the "real" economy to earn money.
Although it can be beneficial, it can also be unfavorable. The more people are being paid under the table without taxes, the less money the government has to spend. Government programs depend on tax money. With less taxes being paid, the higher they will be so the government can do its job. In addition, people with jobs in the secret economy do not get Social Security or health benefits. There is also a higher risk of losing your job and not getting paid at all.