I've recently started working on an application using email and was considering using Nylas but have hesitated for the same reasons as you. But IMAP is such a pain that I may go ahead anyway. Have you considered running their sync engine yourself?
Any chance you could tell us what the pain is that you're experiencing with IMAP? I don't think I've ever had any problems with it, aside from offline (but for that there's POP) so I'm interested to hear your experience.
There's two things primarily that burned me a few years ago when I was working on an MUA:
1. Message Identifiers/Primary Keys - if I recall correctly, IMAP doesn't have a way to definitively map a message and a unique identifier to the message in a way that is guaranteed to persist long-term.
2. Not exactly IMAP, but malformed messages. When I was working on this, I was using my GMail corpus, which goes back to about 2006 or so. Somewhere around 30-40,000 messages. Turns out it was a ridiculously good test case for parser robustness. Messages that claim to be ASCII-7bit, but have extended characters. Messages that claim to be UTF-8 but have invalid codepoints. Messages with malformed MIME headers. Messages with a text/plain section with garbage in it, while having a nicely formatted text/html section. Messages with embedded CSS that tries its hardest to blow yup on-screen display.
I think that through the duration of the project, I only had to submit 2 PRs to the upstream net::imap ruby library, but damn were there a lot of heuristics in place to try to successfully grab an email from IMAP and display it on screen.
My app does a lot of folder/label management on the user's behalf, and some of the issue I've run into are:
* UID MOVE not always being supported (meaning you need to copy and expunge), and weirdness around gmail's IMAP extensions. This could be abstracted away with a good high level library, but I haven't found any for Ruby that covers all the cases properly.
* Gmail offers OAuth, but for regular IMAP, needing to store user's email and password and the security risks that come with that.
* Needing to know when new messages arrive, so either polling regularly, or keeping a bunch of connections open and using IDLE.
Nothing unsurmountable, but using a service like Nylas means I can spend more time developing features instead.
What I mean is, Nylas takes care of asking for and storing the user's credentials to their email account(s), all you need to store is the token that accesses the Nylas API.
