The cryptography is sound, however, it's also frequently changing, in addition to straying from standards more or less. This makes it difficult to give a firm answer.

This ETH (i.e. Zurich) paper[0] identified several exploitable vulnerabilities (bad), which were quickly addressed by delta chat (good).

So overall, I'd see it as a good messenger, but with downsides.

[0]: https://www.usenix.org/system/files/usenixsecurity24-song-yu...