I'm not that familiar with Plaid, but if it works like Yodlee, users have to hand over their credentials so there's no real security, it's just that their scraper is designed to be look not touch.
Plaid has OAuth-based access for most of the big institutions now, but yes, for smaller institutions, they do scraping. Thankfully, Plaid's been around for a while now and has a good track record. It would be a non-starter to give your credentials to a small startup directly.
