The last major outage for S3 was something like 4-5 years ago, IIRC. Coupled with CloudFront or another CDN, your site probably would be extremely resilient to traffic spikes or hardware/datacenter issues.

For elections 2012, we actually used S3 buckets in two different availability zones and CloudFront to abstract away the backend.

In the event that we lost a geographic region, we planned to switch to a different S3 bucket.