Jon Christian of the Outline tried jettisoning Google for the secure confines of ProtonMail, only to find a poorly-functioning search feature ostensibly because of encrypted messages. And:
It’s worth noting that the lack of a robust search feature isn’t the only criticism of ProtonMail. Whether it provides better security depends largely on what sort of threats you’re worried about. While ProtonMail might provide some advantages to people who are concerned about government snooping, Google’s vast expertise and computing infrastructure arguably mean that it’s better equipped to defend against garden-variety hackers.
Even if you were concerned about state actors gaining access to your email, you may also be better served by a company as large as Google or Microsoft. Although they do grant many government information requests, both companies also have teams of lawyers who fight back.
In any case, email simply isn’t secure. If you’re worried about an advertising company holding onto your email, fine: use ProtonMail or, if you need full message search, try something like iCloud Mail, FastMail, or even hosting your own. If you’re worried about hackers, a bigger company is probably going to serve you better than a smaller one, assuming you have a secure, unique password for your email account. If you’re concerned about an IT manager at your organization noticing that you’re leaking confidential information to journalists, PGP-encrypted emails might help. If you have reason to believe the government really is snooping your email account, it doesn’t really matter which email provider you use.