Moxie going on twitter to say the cops have broken into Signal would be headline news, at least in the tech world.
Do you really have the talent in your organisation to develop a better cipher than AES or ChaCha? If not, go with something that exists. According to Snowden, even the NSA can't just break PGP if you use it properly.
Do you have better coders than OpenWhisperSystems? You're going to have to trade off relying on someone else's software versus the chance your own coders make a mistake. I'd say the latter risk is usually the bigger one - even the Sony PS developers messed up on the "don't reuse nonces" bit.
Do you have your own chip fab? If not, you're going to have to hope whatever you're using doesn't have too many backdoors.