The exposure
Any provenance layer is one more system to attack, and you're right to treat it as such. Framed as a data platform, it's just more surface, more privileged access, and more liability to own.
How evidence infrastructure answers
Evidence infrastructure is built as a control point, not a store: federated (data stays in place), semantic access control finer than your current stack can express, every decision provenance-auditable, designed to the invariants of mature security theory (Bell-LaPadula, Biba, Clark-Wilson). The audit trail is a structural property, not one inferred after the fact.