Exactly. Under FEIE, up to $126,500 in earned income is excluded, however, the 15.3% self-employment tax is separate tax not exempt by FEIE. ie. if you are self-employed and made $100,000, that full amount of $100,000 is excluded from income tax in the US but you are still obligated to pay $15,300 in self-employment tax, which is essentially Social Security and Medicare taxes. My income tax rate in Argentina was between 25-30%.
Using the above example, If I made $100,000 last year, I paid $15,300 in self-employment tax to the US, and $25,000 in income tax to ARG = ~40% tax rate.
Obviously the Monotributo system has far less tax burden than the upper limits of RI, so depending on your income it may not be as bad as my situation. Living here as a high earning, self-employed US citizen is pretty much a terrible idea (if you play by the rules, that is).