I don't disagree that AWS Cognito isn't the easiest auth service to work with, but once you figure it out, it works just as well as the others.

I've experienced the same exact pains (and many more) that the author described. But the thing is that once you've experienced those pains, you know how to deal with them. In software, you just have to figure it out once and then it's done.

I can't say I scaled Cognito usage to anything massive, but I can say that keeping everything in AWS is worth the hassle (at least depending on the context). Cognito provides plenty of options of actual customization (the hosted UI is only good for initial testing, then toss it). And, of course, LLMs are able to deal with Cognito just as easily as any other auth service. I didn't have the luxury of using LLMs when I set up Cognito, but it's still my go-to for auth and Claude doesn't stumble on it.