I very much doubt that Ryanair is in any kind of financial difficulty.
However, there is a truly vast quantity of refunds to be processed and I suspect that a refund back to card needs rather more manual input than simply issuing a voucher. Having said that, O'Leary has no reason to make the process of a cash refund particularly quick - we do know that the company is extremely effective at making money by offering a basic service with add-ons? I'm just a bit surprised that there is not an option to accelerate your cash refund in exchange for a small payment? I suspect that may be illegal though.