They aren't meant to be, but shitty teams are shitty. You can also create a COE and assign it to another team. When I was at AWS, I had a few COEs assigned to me by disgruntled teams just trying to make me suffer and I told them to pound sand. For my own team, I wrote COEs quite often and found it to be a really great process for surfacing systemic issues with our management chain and making real improvements, but it needs to be used correctly.

At some point the number of people who were on shitty teams becomes an indictment on the wider culture at Amazon.

Absolutely! Anecdotally, out of all the teams I interacted with in seven years at AWS across multiple arms of the company, I saw only a handful of bad teams. But like online reviews, the unhappy people are typically the loudest. I'm happy they are though, it's always important to push to be better, but I don't believe that AWS is the hellish place to work that HN discourse would lead many to believe.

I don't know any, and I have written or reviewed about 20

Even in a medium decent culture, with a sample of 20? You know at least one, you just don't know it.