You know when it breaks (built in monitoring) and there are SLAs. If it is broken Amazon will fix it. I can work around it, using caching proxies for S3 for example. There are other approaches to be more resilient against cloud outages. The most important difference between self hosted Kubernetes vs cloud native services is that who will fix it.

If there's a problem with an aws service then aws will fix it. Of course there's always this weird bug that you will run into even in their services, but at the least, if there's an API that's documented, it works as documented for the documented scenario.

An autoscaling group in AWS might have issues if you complicate it a bit and run into edge case bugs, but otherwise it's most definitely going to scale your instances up and down (because they bill you to do so)

What AWS does is easy in the small scale but hard on a large scale. The core tech is about giving you a seemingly unlimited supply of object storage, disks, virtual machines and networks to connect and isolate them. This is something a hundred companies have been doing for years but AWS is in the lead thanks to their level of tech that has made the buying of computer resources fast and easy.

Simply said: buying basic VM/dedicated machines has been around long enough that it is not a black box for professionals.