There are forces pushing for chat control again and again and again, until one day people are not vigilant enough to prevent this BS from happening. Then it will take ages to repeal and repair the damage done. Once there is some chance to vacuum up some personal data, businesses, government, and security forces will be very quick to grab that chance, and very slow to give that data up again, should it turn out, that the whole damn thing was illegal all along, due to some constitution. The malpractices might continue for years even after things have been declared illegal.

Not to forget the European Human Rights Convention as interpreted by the European Court of Human Rights (ECHR), although it's not an institution of the EU but the Council of Europe, spanning all EU states and then some.

The Court's case law is recognized as is by EU, and includes some landmark privacy rulings, e.g., against access to contents and metadata of electronic comms without a court order (or comparable legal safeguards), or any forms of bulk surveillance.

https://www.patrick-breyer.de/en/chat-control-2-0-trilogue-u...

> given the constitutions of various EU countries

EU law superseded any state law by default and that's by design. Even if the EU law is immoral and dangerous like the Chat Control law.

So a country's constitution will not save you if tomorrow all chat apps are doing client side scanning.