So when you're looking at a new encrypted data service with their own DIY implementation of cryptographic protocols, this right here is the most important question:

https://delta.chat/en/help#security-audits

This is good. It has been almost two years since the last audit, which probably encompasses a lot of software changes. But having any audits, and discussing that up-front in the FAQ and including documentation is awesome. It's good to have alternatives available.