Zelle has been a nightmare to use. Apparently they rolled out a major update that caused many financial institutions to drop support. Because of that, my phone number is now unusable/blocklisted in their system because it's in a limbo state that neither Zelle nor my bank can fix

The Federal reserve banks are only one operator of ACH, with there also being a second private operator. The fed denies that Fednow will replace ACH.

Not sure if that denial makes much sense, although it certainly could take a long time to replace ACH even if it eventually does, simply due to how many systems interact over ACH, and that many of them will not be high priority to change.

I'm also a little surprised that FedNow went with real-time gross settlement, simply because that means posting every transaction to a Federal reserve account (which would be a large increase in transaction volume for those accounts, relative to say daily or even hourly net settlement). Reading Operating Circular No. 8 tells me that is exactly what they are doing, which is honestly a little impressive.

It surprises me because in Canada the Interac e-transfer system has been flawless in my anecdotal cases. Albeit one known issue is reversals allowing fraud, but I assume any non-cryptographic solution will have the same issue.