APUs have a common mode failure with main engines: lack of fuel. Are there designs out there which require emergency power after engine failure and don’t have a RAT?
There was a Mentour Pilot video on the Jeju Air crash that touched upon this. With a dual engine failure on a 737, you lose both generators and fall back to the emergency batteries (no RAT). The aircraft remains flyable, but it's extremely challenging - you need an operational APU to make things a lot more manageable: https://youtu.be/9GbmGUk8Y0M?t=2001
