And how reliable is it?

There have been reports about S3 returning 50x errors, when (as documented) the client should retry. A browser is not an S3 client, so it won't retry ... https://news.ycombinator.com/item?id=4976893 https://news.ycombinator.com/item?id=4977360 https://news.ycombinator.com/item?id=4981482 https://news.ycombinator.com/item?id=2897959

yes but the recommended approach is to put CloudFront in front of it, if you care about performance at all

I think the point of their post, is that this is their "oh shit we are down" setup, which will consist of a small static site that S3 will happily serve up to anyone anywhere in the world very quickly. This isn't meant to be a full wack highly performant copy of their existing website fronted by CDN's.