This kind of misses the entire value of Protonmail - that they encrypt all of your data at rest using your key, not theirs.

> Fastmail’s email search features also allows you to search for emails based on their content or attachments; ProtonMail’s search function doesn’t let you search through email contents unless you ask it to download all your email to your browser’s cache.

That's inconvenient, but entirely the evidence that they're storing your data correctly. If they could build indexes of your email contents, that would defeat the purpose of private email.

At the end of the day, if Fastmail gets breached they take everything. If Protonmail gets breached they take email datetimes and subject lines.