whatsapp and signal have forward secrecy, so if your private key is leaked it means that past conversations can't be decrypted. In reality it does not offer a lot of protection if you don't disable keeping logs (because losing your phone and malware are the only realistic ways of your private key being leaked). In addition the way that they have forward secrecy implemented it means that you have to decrypt every message posted in groupchats while you were offline sequently until the last one, which can take hours in an active (even if small) group if you are gone for a week. The other thing is that both of these apps to my knowledge do not warn you if a new key is added (I might be wrong here) so an active attacker can pretty much nullify the encryption, this is not an issue with openpgp.