Not all FATCA IGAs︅ are bilateral. The IRS announced that they would not engage in reciprocal exchange with foreign︆ countries that do not meet their stringent information safeguard, privacy, and technical standards.
Even with︇ countries that have reciprocal IGAs, the amounts of information exchanged in both directions are not︈ equal. The US requires foreign countries to report names, SSNs, account balances, amounts paid in︉ during the year and lots of other information. All they send back is the information︊ the IRS already collects on foreigners, namely, US-source interest income earned on individual depository accounts.︋ For non-cash accounts, they report US-source dividends and interest earned. The IRS does not seek︌ to identify the residency of beneficial owners.
I don't know what a "John Doe summons︍ for all accounts held in Puerto Rico" is, but it sounds like pure fantasy.