I think trusting your security or privacy to website-based email is a bad idea. If the email is being displayed in your browser, then the authorities can coerce the company that owns the website to include JavaScript in that page that sends the plaintext content to them too -- or demand the website's TLS key and start intercepting the traffic that you see.
The only encryption-based security that you can reliably trust is encryption that happens locally on a device you control, and that doesn't involve a web page or website loaded from a 3rd party.
If you want privacy protection with real end-to-end encryption that the government can't get past trivially with court orders, use services where the decryption happens on devices that you own, such as WhatsApp or Signal or iMessage. If you must use email, do the encryption yourself on a hardened Linux distribution like Tails using PGP for email encryption; but this is much harder to set up than the above secure messengers.
I wouldn't say ProtonMail is a scam, but a trivial software change on their server-side would let the authorities see your email every time you do. If they can be compelled to make that change then the "encryption" you're paying for is worth nothing. The next time you sign in, a court-required modified version of their server software can capture your password, and then use whatever key derivation function gives them your encryption key.
This might not even require the company to actively participate. In the case of Snowden and LavaBit email, the US Government demanded LavaBit's TLS certificate so as to intercept the communications themselves at the ISP layer when LavaBit refused to comply with narrower court orders to provide information about his account.
What could police do with ProtonMail's TLS certificate and court authority to intercept and MITM traffic for your account? They can probably capture your password, use that to read all of your old email, and at minimum read your email as you read it. Even if decryption is happening in the browser somehow with JavaScript, that JavaScript is coming from the origin server that the government now controls by virtue of MITMing the traffic with the site's TLS cert, and so they can insert JavaScript that logs a plaintext copy of either the emails or the encryption key needed to decrypt them.
There is no security with web-based communications if the companies involved can be coerced with a court order. US based firms would be required to hand over their TLS cert if they weren't willing to help track someone, and at that point the government could do anything to your traffic.
The only secure encryption happens on your device with no browser involved.
By comparison, if you're using an iPhone, in theory the US Government could try to force Apple to modify WhatsApp/Signal on your phone, or force the App developers to do so. These companies would all fight tooth-and-nail in court against doing so. Plus, you can configure your iPhone to disable automatically updating apps, so once you have a working version of WhatsApp installed, unless Apple has some backdoor-ability to push an update of it to your phone anyway, you could turn off app update and be cautious & picky about when you choose to update WhatsApp or Signal. What I don't know how to do is verify the integrity of their binaries: to confirm that what you're getting is the same app distributed to everyone. Facebook would appeal to SCOTUS before allowing a government to install a backdoor into WhatsApp; so would Apple, based on their response to the government's request to unlock the San Bernadino shooter's phone.
All that being said, if the government's goal is simply to discover your identity, which was the case here, then Signal and WhatsApp won't help you. Their accounts are based on a phone number. If the govt has your phone number then unless it's a burner acquired with no name registration then they'll know who you are, and regardless will be able to find out approximately where you are, if you continue to use that phone number. They can triangulate where you are fairly rapidly with modern technology, and this is assuming that the cell company can't simply send a signal asking the phone for its GPS-based location; but even if the govt only knows your nearest cell towers, narrowing that down to a building is a matter of minutes once they're in the area.
If you need to communicate in a way that keeps your identity a secret then you're probably best off using a free email service over Tor from a machine running Tails Linux, accessed from various locations that provide public wifi.
