Plaid use screen scraping which is why it breaks all the time. In terms of reverse engineering undocumented but official APIs, you’re thinking of their competitor Teller.

I was under the impression that they've built up much warmer relationships on the financial account side and correspondingly have more reliable integrations than they used to.

I've personally noticed it tends to break a lot less than it used to.

They have better integration with some banks, one which goes through an OAuth flow.