I’ve talked to a deltachat dev about this, and the signal protocol is simply technically superior to autocrypt (used by deltachat, etc.). It gives you many more guarantees about things like forward secrecy. There is standardization work to bring these benefits to autocrypt, but it will probably be a while. Signal also has the benefit of having a familiar setup for non-techy people, so it is easier to get people to use in my experience.

The database shouldn’t have any issues, since it will be getting a salted hash (one would hope). Usually there are safeguards on the auth system blocking that kind of stuff so that people don’t create passwords they can’t type. However, it would be technically feasible to allow those characters in a password.