I may be mistaken, but I really could've sworn that a lot of the really strict SLA guarantees Amazon gives assume you are doing things across availability zones and/or regions. Like they're saying "we guarantee 99.999% of uptime across regions" sort of thing. Take this with a grain of salt, it's something I only half remember from a long time ago.

[–] skuzz@discuss.tchncs.de 2 points 21 hours ago

The problem comes in so many directions in real life though. Say your company has a very large database. Replicating it across regions means you're paying for data ingress/egress and more than one region's copy of that already sharded and/or duplicated database. It even applies when transferring data across AZs in a given region. Backing it up to S3 is expensive, backing it up to Glacier is cheaper, until you ever have to do a restore, and then you have to lay off half the staff to pay for it.

Other issues can arise, possibly through the fault of yourself, sometimes at the fault of Amazon, if data traffic routing has a glitch and data is routing to the wrong place. The onus either way is on your company to show Amazon the receipts if you expect to get credits for the overage. At larger scale, this could be hundreds of thousands of dollars in overage. Easy to torpedo smaller companies with one mistake.

They didn't used to nickel and dime as hard as they do now, which doesn't help, but outside of history, they set up AWS to be the biggest slippery slope of wallet-deletion, as almost every move you make costs money. Entire companies exist to manage your AWS costs (for more money, of course) and other companies' products you may use that are hosted in your infra may accidentally delete your wallet if you don't constantly monitor them.

Using AWS cost-efficiently is only accomplished by ostensibly day-trading your cloud resources like a high frequency stock trader, capitalizing on unpopular/weird system types, and keeping your code as portable as possible.

...but if one didn't care about cost, one would probably get pretty good reliability out of them, sure.