AWS S3 buckets are highly reliable but it is worthwhile to be aware that the contents of a bucket are stored in a single region. Because of this there is always a risk that a regional outage might result in bucket contents not being available to other regions.
A way to resolve this is to use the AWS S3 Cross Region Replication support in S3. This will cause any object stored in a bucket that has been so configured to be replicated to a bucket in a different region.
An example of doing that using CloudFormation to create the 2 different buckets can be found at https://github.com/dougtoppin/example-aws-s3-cross-region-replication
An example of where this might be helpful is in disaster recovery plans where it is necessary to ensure that bucket data is always available even in a regional outage situation.