I don't think .at is mismanaged, quite the opposite, but in the way of many ccTLDs it is a little bit eccentric. I don't know why route53 might not support them

I don't think .at is mismanaged, quite the opposite, but in the way of many ccTLDs it is a little bit eccentric. I don't know why route53 might not support them: perhaps it's to do with contractual faff, or possibly because nic.at uses a less common flavour of EPP. (Sadly the common flavour of EPP has a bonkers data model; a few ccTLDs use an alternative model with a more sensible data model, but although the more sensible flavour of EPP is easier to use in isolation, it has the downside of being different from the ICANN flavour.)