The problem is not RH paying its counterparties, the problem is me, a retail customer paying RH. It takes time for my money to settle with them, and they can't pay their counterparties until it does.

They keep a pool of cash on hand to deal with this, but that pool was not big enough to absorb the GME mania. Robinhood eventually borrowed enough money to solve their liquidity problem, but by the time they did, the bubble popped, and the bagholders started screaming about fraud and collusion.

Few banks support consumer-initiated FedNow transfers. The main near-term application is receiving government payments: Medicare, tax refunds, and such. The US Treasury wants to convert over, to reduce customer service issues. ("Where's my Social Security check?") If it only takes seconds, customer service and actual money transfer are in sync.

Banks are dragging their feet on customer-initiated transfers because of the fraud problem. If you send money by FedNow, it's gone. So there's a lot of scam potential. Wells Fargo supports bill pay via FedNow to known payees, which is useful. Pay a bill, and the payee gets paid immediately. Both sides can see the transaction as completed. That's useful.

The ability to send your entire bank account to anybody in seconds with no way to reverse the transaction is rather risky.