http://www.theregister.co.uk/2015/03/18/is_the_dns_security_...
In light of the "Let's Encrypt All the Things" movement, I'm surprised that anyone could ever support DNSSEC.
Also, some older gear routed/forwarded IPv6 in software; making it much slower.
Our biggest problem had nothing to do with network hardware. It all came down to developers still writing IP regex's wrong for our app. Maybe I should write an e-book "IPv6 for the node/jquery developer?"
tl;dr it's not in Amazon's market interests to delay IPv6 adoption
I have never known anyone working with IPv6 for more than a day who doesn't understand that IPv6 does not have a broadcast address. I do not believe any vendor writing a routing stack would fail to understand that. Though, I do agree in early days, they screwed things up. For example, Dell would not route RFC 4193 (FC/7) addresses around 2005. I had to rip out and ship back a dozen of their routers when I realized they weren't going to fix that.
I think you might be onto something about gear though - I've read that Amazon builds their own routers, and routing stack - which means they can't simply deploy a well tested/regressed IPv6 stack, they have to write and test it from scratch. The dark side of doing everything yourself.