Which exchange is safest to buy and hold on, and which have failed before?

Every exchange is a single custodian; the columns that separate them are proof of reserves, incident history, fees and how hard it is to leave. It depends on what the holding has to survive: a single point of failure can be excellent at everything else and still lose everything at once, so read that column first, then the fees against your size.

Proof of Custody is published by Onramp Bitcoin, which appears in this table and is scored by the same methodology as every other platform.

Facts are from each provider's own documents and public filings with the date the record was last checked; "n/s" means not stated by the provider. Scores are Proof of Custody's judgement on the published methodology. Coverage is selected, not exhaustive; absence is not evidence of safety. As data: /data/matrix.json. Dispute a cell through corrections.