In addition to all the usual reasons you shouldn't store your bitcoin long term with a third party (hacks, scams, insolvency, shut downs, frozen accounts, loss of keys, governments restricting access, privacy concerns, the list is endless), with Coinbase Vaults there is yet another reason to add to the list.
Coinbase ran a multisig vault service for a few years, until they shut down that particular service. There was a user on here a few months ago looking for help on how to access coins he was storing in a vault when they shut it down. As far as I am aware, he never managed to regain access to those coins. You can read the thread here:
https://bitcointalk.org/index.php?topic=5316286.0There is nothing to say that Coinbase won't do the same again in the future and discontinue this service.