I'm not a fan of my email being categorized for me
I pretty much like all things Google, but Inbox just isn't for me. I much prefer good old fashioned Gmail on my iPhone. The way Inbox sorts my emails doesn't always make sense to me, and I suppose I'd rather sort it myself. There are definitely better email apps out there. I know because I've tried a bunch of them.
It simply works.Cons:
It's not smart enough.
With the rise of new and awesome email clients like Polymail or Superhuman, the bare basics sometimes works better for some people like me.