@fnkr I agree that it would be really cool if Enpass were completely open source (or at the very least, the code for the password generator). As someone who tries to do everything using FOSS I'm willing to use it because the core crypto part of it is open source/cryptographer-reviewed (SQLCipher), with some kind of awesome sauce on top to make it user-friendly/cross-platform (thanks to NW.js, I'm guessing?) I guess I also trust them also because their business model is pretty transparent, and the fact that they are ownCloud-friendly.
Have you tried looking at the Enpass app with Wireshark?