"One thing to note is they also encrypt the e-mail subject, whereas (for example) Proton Mail does not." - protonmail does encrypt their mails if you send it to another protonmail address, and skiff is doing the exact same thing.

Proton Mail encrypts the content of the mails, not the subject. So even if it is a Proton Mail-to-Proton Mail conversation the subject is not encrypted. Skiff also encrypts the subject of the mail in a Skiff-to-skiff conversation.

proton-to-proton-mails are end-to-end-encrypted and so are password protected mails sent outside proton servers. i once had to reset the password of my kids protonmail and all the mails - including the subject - had become unreadable.

https://proton.me/support/proton-mail-encryption-explained