For open source messengers, you can check whether they actually encrypt your messages and whether the server has access to your encryption keys but what about WhatsApp? Since it's not open source, you can't be sure that the encryption keys aren't sent to the server, right? Has there been a case where a government was able to access WhatsApp chats without reading them from the phone itself?
Another thing to consider is that the US (and probably most 5 eyes countries) have agencies with a "store now and decrypt later" policy. They theoretically could be capturing certain types of traffic and storing it in the massive NSA fusion centers. If you come under suspicion at some later date and the quantum technology has advanced, you could be hosed. Now what's the legality of storing "precrime material" without a warrant? I wouldn't think it is legal but that doesn't seem to stop the 3 letter agencies these days.