Since banks don't provide secure mechanisms for third-party authentication and authorization, e.g. OAuth, Plaid receives you credentials in plain text and will then use them to communicate with the bank. So you really have to trust Plaid.
The second weakness is even more dangerous: Apps implementing the Plaid authentication flow will show the Plaid "login page" with bank selection in an overlay on their own sites. Since this is not a redirect again, you don't even see whether your credentials are transferred to Plaid or the third-party app. That is, you have to trust your bank (sure!), Plaid (okay!) and the app using the auth flow (dangerous!).
You should fix this!