Yes, now you mention it, the absence of Delta Chat in the Debian repositories and the total absence of any way to verify downloads for desktop is an adoption blocker for me on desktop. (That’s why I’m sticking only to Android for now.)
Another adoption blocker (or near adoption blocker) is that Delta Chat allows new contacts to send you unencrypted messages before E2EE is established and there is no way to prevent this. The warning about unencrypted messages is not even very noticeable, and many new users will overlook the warning or fail to appreciate the significance.
Honestly I feel a bit silly when I tell someone that Delta Chat is an encrypted messaging app, only to contradict myself when they scan my QR code by explaining that messages are not actually encrypted at first and they need to wait before sending me a message.