Most accounting-firm document vaults do not fail at the bucket. They fail at the edges: a download link that outlives the engagement, a KMS key policy as wide as the bucket policy, a retention lock that cannot be undone, and an audit trail nobody switched on. Six failure families, and the control that closes each.
Shared admin credentials work right up until someone resigns or an auditor asks who did what. Here's how to design client AWS account access around cross-account roles, unique external IDs and per-engineer attribution, so twenty accounts need zero shared keys and revocation is one action.
Hand-built client servers fail quietly: you only find out they were never reproducible on the day you need to rebuild one. Here is how to automate server provisioning for new clients using cloud-init for first boot, Ansible for convergence, per-client secrets and a verification gate that refuses to hand over a half-built box.