Apart from the other issues mentioned above by @WofWca and @r10s, Delta Chat unfortunately makes it very easy to shoot yourself in the foot by accidentally sending unencrypted messages due to the “opportunistic” encryption. This can be mitigated if/when DC implements forced encryption (or at least the option for forced encryption).
The message padlocks are so small that you need to be both an advanced and vigilant user to know if you are sending encrypted or unencrypted messages, as I don’t think Delta Chat displays any warnings if you’re about to send an unencrypted message.
Moreover, you can even be caught out when you scan someone’s QR code. After you have already scanned someone’s QR code, you might reasonably assume that E2E encryption should apply, but this isn’t actually true until both parties come online and do the hidden key exchange. Unfortunately none of this is intuitive or properly explained in the app and even relatively advanced users can easily slip up here.
For example Alice could scan Bob’s QR code, then send him the message “Hi Bob, can you read this E2EE msg?” and only later realize that she sent it in the clear (maybe also compromizing Bob’s identity if he was using a pseudonymous account)!
If it’s expected that Alice has read how the key exchange protocol used by Delta Chat works before she starts using the app, it’s probably fair to call Alice an “advanced” user.