Perhaps this is my bias coming through. I work with DBs day in and day out, and the main problem I face is performance from poorly-designed schemas and queries; next largest issue is referential integrity violations causing undefined behavior. The security issues I’ve found were all people doing absurdly basic stuff, like exposing an endpoint that dumped passwords.

To me, if you’re relying on having a matching PK as security, something has already gone wrong. There are ways to provide AuthN and AuthZ other than that. And yes, “defense in depth,” but if your base layer is “we have unguessable user ids,” IME people will become complacent, and break it somewhere else in the stack.