The whole company is a privacy and security disaster. Of course it’s annoying that banks don’t provide reasonable OAuth APIs, but Plaid “disrupts” that by deceiving consumers into dangerous security vulnerabilities with their most sensitive personal data.
When you use Plaid, you don't get the impression that's what they're doing. We're used to dialogs to "give permissions to an app" that don't share our user/password with anybody. Plaid purposefully emulates those dialogs and gives you the impression that you're just logging in with your bank, instead of explicitly telling you "we will store your user and password and use that to log-i with your bank".
I guess technically they just say, "you will be prompted to enter the username and password associated with those accounts" and don't specify that they (Plaid) will be using your credentials, but I don't think it's clear enough that you are giving your credentials away!
From their website [1]: "When you choose to connect your financial accounts to an app using Plaid, you will be prompted to enter the username and password associated with those accounts. Plaid then links your accounts to the app you want to use so you can share your data."
[1] https://plaid.com/how-it-works-for-consumers/