Because Proton Mail uses E2E and zero-knowledge encryption, there isn't a lot of data that they can hand over to anyone. The only thing that is stored unencrypted is message headers and the email addresses of contacts. Even here, Proton AG says they won't hand over any data unless directed to by the appropriate Swiss authority. Your data is about as safe as it can be using publicly available tech. A bigger risk to the security of your data, is the way governments are pushing to break end-to-end encryption. There are constant efforts to force companies to insert "backdoors" into their software that would allow law enforcement to bypass encryption. This recent Fortune magazine article nicely describes the situation in the United States today.