fbi says they "hate" protonmail (translation: they love it) 😂😂😂

It easily could be, and that's concern enough because you are placing trust in a central point of failure. Also worth noting they do log IP addresses and hand them to LE.

The way Proton "end-to-end" encryption works is they generate a PGP keypair for you. There's no option to use your own. Since they generate all the keys and run all the infra that uses them to encrypt and decrypt emails, there's plenty of opportunities to backdoor it.

Still better than Google, Apple, Microsoft but not as private as the marketing wants you to think either.