Act 60 is fairly limited in scope, and you can only get tax breaks on a couple things. All other income may not be subject to federal taxes, but you will pay Puerto Rican taxes, which may even be higher than federal taxes.
Residents of states also pay state taxes, and in high tax states those can exceed federal taxes so the "they also pay Puerto Rican taxes" line doesn't get them off the hook in any way.
I mean to say state on down, including property taxes, sales tax, gasoline taxes, local payroll taxes. Could be wrong but if you had a house that went up in value over the past few years I could see a situation where the federal income tax you pay is less than your property tax bill alone.
Edit: The more I think about it the very poor pay no federal income tax and the struggling pay very little, they could easily exceed it with cigarette or alcohol taxes for example.
Act 60 seems like a limited tax incentive scheme. I'm referring to the general clause that income of Puerto Rican residents sourced in PR is not subject to personal income tax. From your linked source:
> The U.S. tax code (Section 933) allows a bona fide resident of Puerto Rico to exclude Puerto Rico-source income from his or her U.S. gross income for U.S. tax purposes.