The biggest issue with IMAP is that writing a quality client for it that works on all possible permutations of the spec (especially with the botched Gmail IMAP

The biggest issue with IMAP is that writing a quality client for it that works on all possible permutations of the spec (especially with the botched Gmail IMAP implementation in the mix) is a huge, huge pain in the rear which is why there's a dearth of high quality FOSS mail clients… you've got the old standbys like Thunderbird and Claws and that's about it. Newer clients like Geary either die on the vine or get stuck in a somewhat spartan state because making it more full-featured takes knowhow, time, and manpower that's not generally available. On the other hand lots of devs know how to work with JSON-over-HTTP APIs and a lot of modern languages even come with JSON serialization/deserialization out of the box, with makes writing quality clients a lot more accessible to a lot more people.