The FAQ page for Inbox states "You can use Inbox to replace nearly all the functionality of IMAP and SMTP."

Implying that there is some functionality of IMAP and SMTP that Inbox doesn't handle. Thats OK. In many cases people are willing to lose some functionality to gain other more valuable functionality.

However, nowhere on the website can I find a description of what IMAP and SMTP functionality Inbox doesn't handle.

If I'm going to invest my time in trying out the product, it would be great to know upfront what features it currently doesn't support.

I think the Inbox team would be better served by just being honest and saying "Hey with Inbox you get all these awesome features, in exchange for losing this one feature..."