The KMS autounseal is especially convenient, but you have to know that there is no silver bullet in crypto. You are trading off the convenience of the auto-unseal (and frankly, the fact that this can happen automatically in the middle of the night when your server reboots) against the security of your root unseal key itself.

The only thing protecting the unseal key is access to your KMS. So one rogue SRE can unseal the vault rather than requiring collusion of 1+ SRE members.

Again, this comes down to your risk tolerances and what you are protecting. I think for most workloads, the value KMS autounseal brings is worth the risk, but if you want to have tightest control, then the Shamir Split (M of N) is the best option.