Depends on what you mean by availability. AWS S3 does return 500 errors from time to time and you are supposed to retry. There are also opaque rate limiting. These are mostly hidden from you if you use the AWS sdk but still is something to be aware of if you are using it in scale.

And we haven't even started to talk about eventual consistency.

I am the author of goofys (which exposes s3 like a filesystem) so obviously I believe this can work, but you do need to understand the tradeoffs (or you use goofys and hope that the tradeoffs I've made are correct :-P)

S3 maybe has better availability. Your DSL/fiber and router, that are on the critical path, definitely not.