- if it's an open-source client but it doesn't display fingerprints and you haven't modified it, you're stuck. (At least you know you're stuck, but you knew that already.)

- if it's an open-source client but you're trusting someone else's binary, they can attack you.

- if it's an open-source client but you're not trusting someone else's binary, you're not on the embargo list and so responsibly-disclosed bugs are effectively zero-days for you.

- if it's an open-source client but it's written in C, you have no practical way of auditing it against intentionally-malicious source code (i.e., for almost everyone, the cost of auditing it is higher than the cost of visiting your conversation partner in person).