Mail should be simple, I think the best apps are the lightweight ones that don't require too much work to keep alive, and definitely don't require a backend server process.
Also, Mailbox had access to all your mail, and when Dropbox bought it while having M̶r̶s̶Miss.Rice on the board of directors, well...that just felt yucky.
My simple solution to keeping my mail under control:
ONE filter on the Gmail side of things, called "Good Spam". Any reoccurring email I like, but don't want to keep manually archiving everyday, SKIPS inbox and goes into that folder. The filter is literally a huge list of email addresses from senders. (ex snippet "....email@example.com firstname.lastname@example.org email@example.com Hewlett-Packard@us.newsgram.hp.com....")
Anytime I get an email I don't consider "Good Spam" I unsubscribe from.
Then on my phone, I just get important or new mail.
BONUS from using filter instead of unsubscribing:
When you are in that store and you need that BANANA REPUBLIC coupon at the register, you still have access to it. Open email app, search for it, and use it. And you don't need to keep looking at their emails everyday just to archive it. Or in case of Mailbox, swiping right....thumbs get tired too.
I've immensely enjoyed this new system. I, like you, immediately unsubscribe from things that I don't even care about in the "good-spam" box. Only I am too lazy (or don't care enough) to maintain a huge list of email addresses to filter out. I'd rather err on the side of too much filtering (e.g. not in my contacts) since I still get it on my phone/desktop. If I ever want something to hit me right away, I just add it to my contacts, even if it's a company/"good-spam" address.