Inbox is an attempt to make Gmail easier to use for the average user who has simple needs, so all the "power user" features you want aren't supposed to be in Inbox. If your needs are complicated, you can just use Gmail. Inbox doesn't really introduce any new features that Gmail cannot do, it simply reframes Gmail features in a different UI and workflow. For example, Inbox's "done" action is available in Gmail, but in Gmail it's called "archive". Same with Inbox's "bundles": they're just called "tabs" in Gmail.
As an Inbox user, I'm pretty happy with the state of Inbox. I don't need extra features get my stuff done in Inbox. Porting every Gmail feature to Inbox is missing the point.
I only need signatures in my non-gmail account. This is almost the only reason I switch to gmail now, when I have to send an email to someone outside my institution... it's very frustrating that they implemented signatures but only for gmail accounts.
Inbox is in a weird place - the features we want in Gmail are in Inbox. The features we want in Inbox are in Gmail. The mails are in the same place. The web clients are separate codebases. I can't imagine all the talented people working at either team are happy with this situation given the way their contributions end up not coming to us users because of this separation. If anyone here is on either team, or has some insight into this, I'd really like to understand what the thinking is behind this stuff. The same sort of goes for Allo, Hangouts, etc. What gives, Google?