I think it's still a reduction in risk overall. In the old model, they were vulnerable to S3 failing in one region, a thing that's happened many times. Now they've mitigated the S3-failure-in-one-region issue, at least mostly (though as you point out, how they do so is unknown), and in exchange they've picked up a dependency on Lambda@Edge. But Lambda@Edge, like CloudFront, is a global service distributed across many regions, and to my knowledge AWS has never had a global Lambda@Edge outage.
It's not impossible, of course. Some kind of control plane error could probably knock the whole global service offline. But I'd rather bet on a multi-region service than have all my eggs in one regional basket.
It's not impossible, of course. Some kind of control plane error could probably knock the whole global service offline. But I'd rather bet on a multi-region service than have all my eggs in one regional basket.