It's because Ultra lets you do geo based routing by state and province and Route53 doesn't (at least not yet)
I would imagine they don't use it for exactly that case. Imagine a case where most of AWS is having critical problems, you would want (at the very least) control of your DNS to attempt a partial recovery of service for some portion of your customers that have providers who respect DNS TTL's.
Running my own DNS servers. But having issues with clients using Rackspace Apps - hosted email (still down due to UltraDNS).
Would there be anything against using multiple large anycast DNS providers?
Netflix is 100% dependent on AWS for infrastructure. If AWS is having problems, there is nowhere to redirect to.
of course there is, they could redirect to a page explaining the problem instead of a generic error page...
UltraDNS is more advanced / pro than route53.
Could you provide some details about how and why it's better?
Amazon.com uses UltraDNS and Dyn
Confirmed, this issue more or less took down Amazon's Merchant Web Services API today.