Thank you for the link -- is a very informative post. My small organization recently adopted (and loves) FrontApp, it's been a tremendous upgrade over trying to handle inbound support via shared inboxes.
That said, it's pretty darn expensive & it's probably "overkill" for us -- we only have ~5 team members. By the time we added in all our forward-facing "aliases" (ie support@gameX, support@gameY, media@gameX media@gameY, etc) plus a few Twitter accounts, our bill ends up around $240/month.
Comparing the two services via the [relatively] detailed comparison breakdown at your link, it sure seems like Missive would scratch the same itch that FrontApp does for us currently -- possibly better even, especially for a small team like ours -- and looks like it'd be ~50% cheaper than FrontApp.
Will definitely be taking a closer look in the future; I don't look forward to the migration (though sounds like it can be done pretty smoothly), but it's certainly an attractive product and the price point makes it quite tempting.
We wrote this post explaining some of the differences: https://missiveapp.com/frontapp-vs-missive