The main problem is control ig. On Signal, someone can ask for a code or passwords to log into your account or get your data. If you have your own solution, you can have physical security keys to verify yourself, making it impossible to give anything to anyone via the internet. You can also monitor logins and make logins on new, unauthorized devices impossible.

Encrypting stuff is not really the hard part of keeping oblivious users safe. As far as that goes, they will be fine if they have people who know what they are doing use established, well audited implementations.

Yeah because the Polish government cannot possibly create a secure messaging app of their own.

You know before cryptography was a software feature it was a crucial part of statecraft. IMO there is nothing wrong with states building their own secure communication software. It has more precedent than "download a US app"

that may or may not have an NSA backdoor

does.

IDK about Poland but in Germany I know they just forked matrix and basically did a reskin of I lt afaik