I mean, if you're an auth expert, that's awesome; but I think it's the absolute height of engineer hubris to pretend that the efforts of any one individual, no matter how talented and experienced, are able to match an entire organization of people who do nothing but this one thing.

Now, trusting this particular third-party? Definitely a big question mark! It's on them to earn your trust. But I think to say that third parties in general can't be trusted with your users is to be a little ignorant of the modern web engineering landscape. We trust a lot of people with a lot of things, and not in an unreasonable manner.