I'm navigating - or fighting - that right now. I think an IRA should be held in a multi-sig with the owner having 2-of-3 keys. That allows the custodian to still have visibility into whether those BTC move, but "your keys/your coin" so you can't get rug-pulled.
My IRA is with Swan, and they dodged bullets with both Prime and Fortress Trust. I know they have other partnerships now, but it is all 3rd party risk.
However, it is Swan's opinion they must hold all the key to keep the IRS from defining it as having "unfettered access" to move the BTC...even if you have auditability via the timechain. Unchained does allow the multi-sig...but they might be making assumptions, based upon no perfectly aligned legal precedent...that the IRS will not view the Bitcoin like they did the gold in the McNulty case.
Having said that, I think Swan will now support the "checkbook IRA" setup...but they basically say if the IRS comes for their 10% penalty + capital gains, that is the risk we take to have the quorum on the multi-sig keys to prevent rug pull.