..Personally, this looks promising. I am going to say it though- Skiff needs some sort of mechanism to use PGP if for nothing else, then to communicate with Protonmail email addresses specifically. I see they are taking the stance its's time to move to something beyond PGP- but given the extremely large userbase that is Protonmail- I think their target market would feel better being able to communicate with their protonmail contacts- using PGP.
Also, i'm having trouble seeing if Skiff has a way to 'send secure emails' to other external email providers, like Tutanota ,Mailbox, and Protonamil do for their users to securely contact people outside their ecosystems(like peeps with gmail). Solve those issues- and the pain/difficulty/unease of convenience will disappear for their market which likely already uses Protonmail/Tutanota, etc. And Skiff would be very attractive at that point, positioning itself as a successor to those two possibly.