I just use DC in one single chat with one person.
Transmission confirmation is reliable.
I don’t receive reliably a read-confirmation from the other side. Sometimes a read-confirmation arrives a day after my chatpartner had answered my message (indicating he had read it), Sometimes I never get it.
I checked the email-communication and know, that the corresponding confirmation email did not arrive at my place.
On the other side, I sometimes receive a double confirmation on one single message. It might be coupled with the situation, if a picture was included.
My communication partner uses the same email-provider and a DC 1.20.2 client.