Yes, there are restrictions, but not many. The Internal Revenue Code makes those distinctions. In general, as long as an investor doesn’t purchase life insurance, collectibles, coins, real estate that is designated for personal use or engage in a prohibited transaction involving a disqualified person (your direct ancestors and/or descendants), then an investment is possible.
This still leaves plenty of alternative investment choices. Alto has built tools into its platform to help you avoid prohibited investments, though it's always wise to consult a tax professional before moving forward with an investment.