> when someone finds and exploits a weakness in your library to take my money, I'm screwed.

I'm not sure what kind of exploit or weakness you are thinking about. But the risk of using Bankscrap's gem from your computer is similar to using your bank's mobile app. Bankscrap itself doesn't store any data nor send your credentials to any place other than your bank's API (the same that your mobile app does).

In other words, if you use Bankscrap's gem to build a web app to fetch and store your bank transactions (or someone else's), and your webapp or server gets hacked, obviously the responsibility is yours.