If they have such “security concerns” with third-party clients, a compromise would be to mark profiles using unofficial clients, and make it possible to see what client it is. Because it’s audacious to disapprove of third-party ones while your own lacks features people find important! Such as:

• Allowing an arbitrary proxy rather than just their own solution (because not only is their own solution inferior to some of the more advanced censorship-evading technology, but this is the field that needs multiple options when one stops working. Also if a person uses a proxy for everything else anyway, making them set up a whole separate solution or find someone else’s proxy just for your app is pointless.
• UnifiedPush.
• Allowing tying a desktop client by typing a code rather than scanning a QR code, which is important when registering on an Android VM (because again, Signal just arbitrarily disallows account creation on a desktop, nevermind that most phones are very hard or impossible to make private!)