They are it as far as I've been able to find out.  I researched this same issue 3-4 years ago. The reason they (UBS) has them is because UBS has incorporated & registered in PR as a separate entity (from their purchase of paine webber) and they underwrite & get the special treatment of the bonds which for PR residents are tax free as you've probably figured out. The  investments inside the product MUST be from income producing companies/entities located in PR. This only applies to residents of the island as well. The challenge here is that for you to open an account w/ a resident from PR you would be opening the account in a US subsidiary of the firm you work for for creating the hassle for PR residents to have to file US Tax returns even though the investment is not taxable because the income is generated in PR even if you could use the UBS funds (which you can't because they are proprietary). It's practically impossible to match because the yield is about 6% & tax free to residents of the island. The risk is that they are certainly concentrated BUT try to tell that to someone who's getting a relatively steady 6% tax free yield w/o having to deal w/ US Tax returns and you get nowhere.