Answer
It can be, but the safety comes from the seller's process, not from Power9 itself. Ask for a documented test report covering burn-in and diagnostic testing, confirm the exact firmware level installed, and get warranty terms in writing that cover parts and labor, not just a replacement credit.
Buyers should also weigh remaining parts availability into the decision. A refurbished Power9 unit backed by a real warranty and an active third-party maintenance option is a reasonable purchase for many workloads, even after IBM's own standard service window has closed.