The answer is, overwhelmingly, insurance companies peddling high-cost, tax-deferred variable annuities with crappy proprietary mutual fund options and high-risk fixed accounts.
Indeed, mutual funds that used options for either speculation or hedging from July 2003 to June 2007 showed no sign of market-beating returns after adjusting for risk, according to a working paper by Gjergji Cici, an assistant professor at the College of William and Mary.
However, in most cases, the financial institution sponsoring the solo 401(k) plan documents will limit the individual investment options to stocks and mutual funds and will also exclude many of the attractive solo 401(k) plan options, such as the loan feature and the Roth contribution component.