I've a system that has for years swapped NACHA files with a major bank for hundreds of thousands of accounts each day, and without fail it has worked 100% of the time! Guess what we use? SFTP whitelisted to specific IPs. A fixed width GPG encrypted file. That's it.

Can't say the same for most of these rubbish REST of an APIs out there that I've had to suffer through.

Some 70s technologies btw are great. There's nothing wrong with a well defined fixed width file with checksums. I wish all data files were those or CSVs.

Which is not part of the ACH spec... though I agree it is a good implementation

In other parts of the world you can transfer between accounts in almost real time, nobody uses checks anymore for rents or interpersonal transfers or intercompany money transfers.

Meanwhile the US still uses checks. ACH is jurassic.

Real-time transfers under $5,000 are free between most American banks. For real-time irreversible transfers up to any amount, the Fedwire system is available. Most banks let large account holders send and receive domestic wires for free. ACH is used for low-cost, gross-settled transfers--it is the cheap, reversible option.

This myth of slow, expensive bank transfers dies slowly.