Validation of new peers is a notoriously nasty usability issue. When both ends have access to the device contacts, then, in cases that matter the most (when people already know each other), it can be done transparently, because both sides will have each other on their contact lists. The friction is zero. No access to the contacts = the need to manually sort legit contact requests from spam. Hence the question. What is it like with Tact?
Also, as others have said, the fact that all goes through Apple makes Tact no different from iMessage. Perhaps I miss something obvious, but I literally see no reason to switch.
Additionally, taking Apple's privacy claims at their face value is a very naive thing to do, leave alone basing your product on them. At the very least whatever is getting stored in iCloud needs to be encrypted with the key that doesn't leave the device. That would make a good difference worthy of a switch - an iMessage clone running on Apple's infrastructure with Apple excluded from the loop.