And even then, I'm not sure it's apples to apples, at least if by Rego you're thinking of OPA. CEL and Rego take very different approaches, with CEL being quite procedural, while Rego is about constraint satisfaction, not unlike Prolog. At $WORK, Rego (in the form of OPA) gets used quite a bit for complicated access control logic, while CEL gets used in places where we've simpler logic that needs to be broken out and made configurable, and a more procedural focus works there.

Rego is much more powerful, and can do things cel can't.