Does a successful spreadsheet import prove migration readiness?

No: test the specific import route and the records it omits. IBM’s OpenPages 9.1.x FastMap documentation says that FastMap cannot import attachments or signatures and cannot preserve system creation-date information through that process. This is a limitation of that documented route—not proof that all alternative migration methods are unavailable. (ibm.com)

For a ServiceNow IRM vs Archer vs IBM OpenPages evaluation, our proposed test is identical: migrate a small, deliberately difficult record set containing a historical approval, restricted attachment, retired owner and cross-entity relationship. Require each supplier to explain its supported method and unresolved gaps. Do not score a CSV upload as a complete exit-and-replacement rehearsal.